Additional suitcase bombs, one at a house near the airport where this day of horror began. Tonight police are hunting this man, as yet unnamed, seen in the airport with two others. Those men officials believe blew themselves up moments later. The check in area filled with smoke. Baggage and childrens strollers abandoned. A woman in shock holding her child, cowering in the corner. Lots of people were covered in blood. Everyone is crying. And i just yes, its the first time i saw such a thing in my life. Reporter many injured, bleeding, and many dead, killed by bombs packed with nails, isis claiming responsibility for the horror. Outside, shattered windows and fractured nerves. Hundreds fleeing onto the tarmac, smoke billowing from the airport building. You just feel like a wave. Reporter then an hour later, more carnage. A subway cars doors blown out just as the train was pulling out of the station. Passengers escaping aof long the track while heard along the tunnel screaming. Everyone was really shocked. They couldnt even describe because i asked many times what happened what happened. And people couldnt really they were shock and scared. And i said okay so someone said there was a bomb there was an explosion. Reporter americans were among those injured in todays attack. And some still missing. No one has heard from stephanie and justin schultz. Their family increasingly worried. And tonight, those behind the attacks may be on the run. A house raid discovering improvised explosive devices, chemicals to make more bombs, and an isis flag. Tonight a huge manhunt is under way. And this afternoon there were reports of lines of cars at the border undergoing security checks. But there are increasing criticism, lester, or the security failures that some people believe have occurred here. And when we crossed the border tonight, we saw no checks, no police. Lester . Some powerful images and stunning sounds there, keir, thank you very much. There have been no reports of any americans killed in these attacks. But a number of americans were injured including a group of mormon missions. Nbcs Miguel Almaguer has more on the americans in belgium and their families anxiously waiting for word back home. Reporter at least six americans were injured in the attack. But thousands are in belgium. Here at home, their families waited desperately for word from their loved ones. He was there in the middle of the blast. Reporter chad wells son mason suffered Second Degree burns and was hit by shrapnel. The 19yearold was at the airport along with mormon missionaries richard norby, joseph emfy and fanny rachel clain. All are recovering. Ironically mason was in paris when 130 people were killed. And also survived the Boston Marathon terror attack. I think that experience of the chaos and civil unrest prepared mason for whatever he experienced today. Reporter for parents, news came painstakingly slow. Lauren clearey, monica hall and kate duffy, students from quinnipiac university, were also at the airport during the attack. Janet clearey got the call at 3 00 a. M. That lauren and her friends were all safe. This morning she called crying hysterically that there was a bombing, that she was at the airport, and a bomb went off. Reporter some 1500 students study in belgium. Universities like wisconsin, illinois and northeastern thankfully tweeting everyone is safe. Meg hilling is with a group of 16 journalism students at missou. I do feel safe to an extent. I mean, its made me realize that anything can happen anywhere. Reporter tonight most americans in brussels are safe. Their families thankful and praying for those killed, injured and still missing. Miguel almaguer, nbc news. And Airport Security here at home is being examined through a different lens tonight after the brussels attack. As we had said earlier, the ticketing check in area is part of the airport where the public and their belongings are not normally screened. Typically in american airports nbc tom costello is working his sources. Tom, is there a rethinking now of maybe the whole airport needs to be secured . Reporter well, i think that that would be an ideal situation, lester. But the challenge of course is you simply dont have enough people to do that. And you dont have necessarily the money to do it. There is a reality here that there are tight budgets across the country. You know, theres been a great deal of concern all along that if there is a soft target, if you will, at the airport not that the whole thing isnt a soft target but they are particularly vulnerable between the parking garage and the checkpoints. Some Security Experts say listen, what really needs to happen is a Robust Police or military presence in that zone between the curb and the checkpoint. And the more officers you have, heavily armed officers, whether its in europe or here in the states, that in and of itself seems to act as a deterrent in addition the more people who are speaking up and saying something that looks like its out of the ordinary, that also helps. A couple of other moving parts i do want to pass along to you. The embassy in brussels, the u. S. Embassy, is now asking u. S. Government employees not to come to brussels for at least a week until the 29th because of the situation there. And American Airlines is saying that it and other airlines are certainly not going to fly in tomorrow. The airports closed. Thursdays out of the question. Friday looks iffy. But that would be the earliest at best because of the situation at the brussels airport. The forensics investigation and the rebuilding. Here in the u. S. Homeland security is also saying it has no credible information right now about any potential threat against u. S. Airports but it takes all threats very seriously. Back to you. Tom costello in washington, thank you. The deadliest of todays attacks in brussels was the bombing of a subway train. Security experts say that reflects a sobering reality that while air travel is the most heavily protected, the simple fact is more people are killed in terror attacks on the rails. Our justice correspondent Pete Williams has more on the stepped up security in the u. S. Cities. Reporter in new yorks times square, extra police swarm in with 50 officers deployed to the subway station there. On bridges and tunnels, moran dom checks of cars and trucks. On commuter train systems from los angeles to washington, a scramble to turn out a visible show of force. Nationwide, fbi agents are stepping up surveillance of terror suspects on the lookout for possible copycat attacks here at home. U. S. Officials tonight say theres no known threat of a brusselsstyle attack here. The experts say the security picture here is far different. Police from europe are overwhelmed trying to track the nearly 7,000 people who have been trained with isis compared to the roughly 200 who have gone or tried to go from the u. S. Some of the countries in europe have a unique challenge. I think its probably because of their close proximity to some of the countries of concern. Reporter and the fbi has proven adept at stopping potential attacks here. But u. S. Officials say they face one big challenge their european counterparts do that makes terrorists harder to track. The use of encrypted apps, disposable phones and other ways to communicate that cannot be monitored. And a former Homeland Security secretary says the u. S. Has a head start over much of europe, working hard since 9 11 to prevent terror attacks. The bombmakers, however, are very crafty. And they keep adapting, too. So this is not a static situation. It will keep moving. It requires United States security to keep improving. Reporter and one other factor american officials have been working hard to develop and maintain good relations with the u. S. Muslim population whose members often provide a warning of potential trouble in their own communities. Lester . Pete, you mentioned copy cats. How concerned are u. S. Official that is what happened in brussels might trigger something here . Reporter its something they want to watch for. But their general view is that people here in the u. S. Who are going to carry out terror attacks are really on their own wavelength and they dont take their cues from overseas if theyve already got some plot in mind. But nonetheless, they want to make sure that Something Like this isnt a trigger point for something here, lester. Pete williams, thank you. Lets bring back michael lither former director of the National Counterterrorism center. Now president of lydos which does Security Work for the government. Michael, what are you hearing from the Intelligence Community after these attacks about what chatter, what things may have raised the hairs on the backs of next before today. Counterterrorism officials have been incredibly nervous since paris and San Bernardino. Its generally because they dont think theyre seeing everything. They understand how big this radicalized population is in parts of western europe. They simply arent seeing the communications. They dont understand the networks in a way that they have previously. So they feel more blind than really they have for five or ten years. Thats got everyone nervous. They dont see the same activity in the United States, but the old saying in intelligence goes, you dont know what you dont know. These pictures are so chilling. And it makes us all feel vulnerable here in the u. S. But the simple fact, are they fighting a different kind of war against isis in europe than what weve seen . Even as we look at the San Bernardino attack . It really is different, lester. What we see in western europe are largescale sophisticated networks where there is training and coordination to make the sorts of ieds that were used today. In the u. S. It has been much smaller, homegrown inspired, simply using small weapons. They can obviously kill people but generally can be more easily detected by officials. So europe has a larger problem, and they really have a more sophisticated problem than we have in the u. S. Here today. Our keir simmons was on a short while ago reporting they found what may be a bomb factory. Talk about the explosives. Are those sorts of things as easy to get in this country as they might be in europe . Well, regrettably the precursors for these sorts of homemade explosives are generally available. Often its Hydrogen Per Oxide and other readily available tools. Making those weapons is a bit of a challenge. In most instances like we saw in one attack in london in 2007, the bombs dont work. So i think what we have in belgium is again, a group that has been well trained, possibly in syria, and is practiced enough that theyve gotten it right. So far we havent seen that same skill here in the u. S. Michael always appreciate your perspective. Thanks so much. Todays attacks in brussels are having a deep impact here at home on the race for president. The candidates are reacting to all this. And theyre reacting in different kinds of ways. While the man they all want to replace, president obama, is taking heat for his response to the attacks while on his trip to cuba. Nbcs Kristen Welker is on the campaign trail and tells us more. Reporter as terror again overshadows the campaign trail, the sharp contrast between the two front runners taking center stage. Donald trump calling to close up u. S. Borders and other extreme measures. You say you would even go further than water boarding . Is that right . Absolutely. How far would you go . Id go further. Reporter Hillary Clinton calling that unrealistic. You tell me how high does the wall have to be to keep internet out . Reporter for trump, an opening to answer critics who say hes not president ial. For clinton a chance to showcase her Foreign Affairs experience. It will revert back to National Supreme couecurity. Reporter meanwhile the gops number two ted cruz trying to match trumps hard line calling for police to patrol neighborhoods of american muslims. Todays attack underscores the need for a strong commander in chief. Reporter a position that drew criticism from republican and democratic candidates. Because you happen to be of another religion that somehow you have evil intent is just irresponsible. That would be unconstitutional. It would be wrong. Reporter meanwhile, the current commander in chief addressing the tragedy during his historic trip to cuba. And this is yet another reminder that the world must unite. Reporter and despite criticism from republicans, he made no apologies for attending a baseball game with his cuban counterpart. They cannot defeat america. Reporter and now defending america in sharp focus once again. Kristen welker, nbc news, seattle. Still ahead tonight, inside the tunnel as passengers flee for safety and terrifying moments after todays train bombing in brussels. Rrifying moments after todays train bombing in brussels. We are hearing incredibleri we are hearing incredible stories of survival in the wake of the terrorist attacks in brussels. Evan lamos, a new jersey native. Was on the metro train behind the train that was hit by a bomb blast this morning. What followed what a frantic evacuation of passengers through the smoky tunnel. Evan caught it all on camera. Here here is in his own words. This morning i was on my way to work. I went via metro. I was reading on my phone about some explosions at the airport. Immediately i knew there was something going on in the country. Then suddenly there was a kind of blast of air and my ears popped. We heard some loud thuds that seemed to be in the distance. And the metro stopped. And there was an announcement that came over the speakers saying that there had been a disturbance on the metro line. In that time people were a little bit nervous on the metro. There were people starting to talk about the fact that there had been an attack at the airport. People seemed to be a little bit disturbed and a little bit concerned. They opened the very last door and saw the ladder and had us evacuate out onto the metro tracks. There was smoke that collected in the metro tunnel. So as we were exiting the metro it was clear that something severe had happened. When i realized that there was something really going wrong, i had no idea thought was as close as it was. I thought it must be pretty far away and theyre just trying to be extra careful and evacuating us on the tracks. Im still waiting in a sense for the emotion to hit. I know that i was close to something really horrible. But it hasnt really entirely sunk in yet. These people who have been injured and who have died today have died in a way for our freedom. And i think we should honor that by not letting it strike terror in our hearts. Evan lamos in his own words tonight. When we come back, how will this day of terror in brussels Impact Security around the world . Were back with more of our breaking News Coverage of the terror attacks in brussels. We want to bring in we want to bring in now nbc news analyst christopher dickey, world news editor or the daily beast based in paris. Christopher after the paris attacks there was a lot of soul searching about the open border of the e. U. , about security cooperation. What happened to that conversation . And assuming its picked up where does it go now . Well, it was and is an ongoing conversation. Theres always talk about how you can have better intelligence sharing, how the police can Work Together better. But were talking about 28 Different Countries here with very different cultures, very different legal cultures and police cultures. Its very, very hard to get them to coordinate. The idea that you can open up all the borders of europe but not have a single strong federal police force and federal Law Enforcement system was always a fairly impractical idea. Now the chickens are coming home to roost to use a terrible cliche. We talk a lot about vulnerability today. But is there a higher sense of it given the fact that in many ways they saw this coming . The climate was ripe for this. And yet it happened. And even with knowing waits going to happen they couldnt pinpoint it . Well, thats right. I mean, you get this chatter. You hear that somethings going to happen. And frankly just the mood in the Intelligence Community was that things inevitably were going to happen. In fact, on friday morning i was tal talking to one of the french counterterrorism specialists one of the top guys. I said when do you think the next big attack is going to be . He just looked at me and said soon. It was always known there was going to be some kind of attack that isis was going to try and strike back, not just for the arrest of this guy on friday but because this is war that they are waging against the United States and europe. All right. Christopher, good to talk to you. Thanks so much for being with us tonight. Were back with some final thoughts on this tragic day in brussels. Finally tonight, the horror we feel after the brussels terror attacks is sadly become part of a new normal. Maybe its not even new anymore. And yet even under what some call a constant threat of terror, the world in which we live must still find a way to carry on. Heres our harry smith with some final thoughts. Reporter so this is it, folks. Explosions rocking the airport and the city subway system overnight. Reporter we click on the tv in the morning to see whats new, and we are stunned as we watch the coverage we connect our own dots. The arrest last week of the terrorist in belgium. The guy who got away from the attack in paris. We remember belgium, that neighborhood molenbeek. Did the authorities not turn that place inside out . If they did this is worse than we want to believe. The numbers, the resources, the commitment to wreak holy hell whenever possible. We hear the eyewitness accounts. The shock wave that hit just was like nothing ive ever felt before. Reporter just another day stories turned into tales of survival. Our guard is up again, just like every other time this happens. California, paris, we hold our loved ones closer and realize this feeling will fade in a few days. But we also now know this will happen again. Harry smith, nbc news, new york. So let us honor the strength and resilience of this day, too.