Their hair. Nightly news begins right now. Announcer from nbc news World Headquarters in new york, this is nbc nightly news with lester holt. Good evening, im kate snow. In for lester tonight. Its being called unprecedented, the massive shutdown of the Second Largest School District in the nation because of a credible, series threat of a large scale attack, or so it was first described. Over 900 schools closed, searched room by room. Hundreds of thousands of students affected. A chaotic day in los angeles. Then we came to learn new york city received a similar threat. Both now being called hoaxes. And new yorks Police Commissioner says l. A. Overreacted. But with the attacks in San Bernardino still fresh in their mind, imagine being the official who has to make that call. We have late details. Reporter shortly before 7 00 a. M. , police were mobilized and the decision was made public. The nations Second Largest School District saying it received a credible threat, abruptly shutting down all campuses just before the morning bell. They said that it wasnt no school due to a bomb threat. Its very scary. Reporter the unprecedented decision is what many woke up to. Breaking news of the massive districtwide shutdown of lausd schools. Reporter the threat came by email. A person claiming to be a High School Senior ready with an army to attack schools with guns and bombs. Los angeles is just 65 miles from San Bernardino, where 14 were killed in a terror attack less than two weeks ago. I think the circumstances in the neighboring San Bernardino, i think what has happened in the nation, i think what happened internationally, i as superintendent am not going to take the chance with the life of a student. Reporter the police chief said the threat was specific to all los angeles schools. The implied threat was explosive devices. The specific threat was attack with assault rifles and machine pistols. Reporter while the chief stood by, the superintendent, privately loss angeles Officials Say they advised against closing schools, saying the decision was premature and a bad move. The coordinated sweep of l. A. Schools for bombs a massive undertaking. Lausd spans 710 square miles with more than a thousand campuses and 700,000 students, many about to take final exams. We have to live through this and live this way. Trying to see if terrorists are going to come and get us. Reporter with School Busses suddenly turned around, many had nowhere to go. Staff and parents complained notification came too late. Tonight, anthony karsen has to explain to his fouryearold son aj why fear has gripped the region and now his school. He doesnt know like whats going on in the world, like with what happened in paris and San Bernardino. Hes just happy all the time. Reporter tonight Law Enforcement officials are still checking campuses for suspicious devices. Meantime a 17yearold student was struck and killed by a truck after he was crossing an intersection. Its unclear if that student was headed to or from campus after the school closures. Kate . Miguel, thanks so much. It appears a new normal is emerging after the attack in San Bernardino, as a majority of americans now say terrorism is their number one concern. And with that as the backdrop, we saw a tale of two cities today, and the very different ways that new york and los angeles reacted when the threats came in. Heres our justice correspondent Pete Williams on how it all went down. Reporter children in the nations largest Public School system in new york city headed to class today, even though police said a nearly identical threat was emailed to schools there. New yorks Police Commissioner said l. A. Overreacted. It is not something we are concerned with. What we would be concerned with overreacting. Well stay aware, well stay involved. But we at all costs cannot start overreacting. Reporter several Law EnforcementOfficials Say the writer of both emails claimed to be a High School Senior who was bullied and claimed to have an army of supporters who planted Pressure Cooker bombs and had assault rifles and pistols. The writer claimed to be a radical muslim but made mistakes in referring to islam, a phony threat, new york authorities concluded, because the same person could not be a student in two places at once. Local and federal Law EnforcementOfficials Say the l. A. Schools chief made the decision and then consulted police, who Officials Say recommended against closing the schools. But there was a unanimous view about how serious the decision was. These are very high stakes. You know, were not making a decision about the color of a car or where were going to eat for dinner here. These are this is the safety of our childr. There is no more important decision. Reporter how to respond in an era of terrorism, and when schools have been the scenes of mass shootings, has become a huge challenge. Schools are experiencing threats locally, from students, nonstudents, as well as s. W. A. T. Ing threats that send shock waves of panic throughout schools. Reporter investigators say syed farook might have been considering attacks on schools. Tonight the fbi is trying to figure out who sent the threats. Theyve been sent to an email Service Provider in the u. S. Thats been popular with on line pranksters. Pete williams in washington tonight, thank you. Its fitting that National Security take center stage as the candidates face off tonight in the final gop debate of the year. Nbcs Peter Alexander reports a highly anticipated showdown is expected on the vegas strip. Reporter the republican candidates are bracing for a freewheeling and fiery slugfest in tonights final debate of the year, the first with ted cruz at number 2, taking shots from donald trump. The Heated Campaign season boiling over as the trump show rolled into vegas overnight. We should have been doing that for the last seven years. I give him credit. Reporter trumps contentious rally repeatedly interrupted by protestors. Several forcibly removed and heckled by audience members, one even yelling a nazi salute. Seil heil reporter in the five weeks since the candidates last shared the stage, the world has changed. Paris and San Bernardino fueling new concerns about terrorism and National Security, igniting debates over Syrian Refugees and trumps proposed temporary ban on muslims entering the u. S. You have the radical islamic extremists, theyre pouring in. Reporter all of it reshaping the race, with contenders competing to show who is toughest. We will carpet bomb them into oblivion. A unique threat. Its not just foreigners coming into the u. S. Its homegrown extremists. Reporter that threat has exposed ben carsons lack of Foreign Policy experience, despite this statement posted to facebook today. I sure hope we get a lot of questions about Foreign Affairs and national defense. If so, slam dunk. Reporter once the establishment favorite jeb bush may be setting a record for futility, his campaign and superpac spending millions of dollars in tv ads but still only posting single digit support. Chris christie has visited las vegas 50 times since june, looking to impress a national audience. The impressions please candidates make may stick for a while. The next republican date isnt until the middle of january, after the holidays. Just two weeks before the iowa caucus. Back to you, kate. Peter alexander out in vegas. As the republicans duke it out, Democratic Front runner Hillary Clinton detailed her own plains for the white house late today, specifically her strategy for taking on isis. As nbcs Andrea Mitchell reports, clinton saved some of her toughest talk for the republicans taking aim at her tonight. Reporter with fear of another San Bernardino or a parisstyle attack in the u. S. , now a top worry for americans, Hillary Clinton took on donald trump and the republicans for their rhetoric, accusing them of becoming a tool for isis recruitment. Promising to carpet bomb until the desert glows doesnt make you sound strong. It makes you sound like youre in over your head. Reporter her plan to stop isis foil plots. Enlist Silicon Valley to attack isiss online recruitment. Empower muslim communities on the front line. But while clinton likes to portray herself as a grown up bluster and bigotry are not credentials for becoming commander in chief. Reporter critics say she too is vulnerable for policies in iraq, libya, and syria that helped spawn isis, and for failing to help counter islamic propaganda online. Plus russias 11week air campaign in syria has complicated u. S. Plans. Defense secretary ash carter visited u. S. Pilots in turkey today, accused russia of bombing the antiassad rebels, not isis. There are those not working in the right direction. Obviously that includes russia. Reporter while john kerry in moscow tried to persuade Vladimir Putin that isis is now a threat to all of the civilized world. In a clear shot at president obama, Hillary Clinton also said its not enough to contain isis, as obama once said. She said isis has to be defeated. Andrea mitchell in washington. High drama in baltimore today. Jurors in the trial of the first of the Baltimore Police officers charged in the death of freddie gray told a judge they were deadlocked and could not reach a verdict. But the judge sent them back to deliberate further. Grays neck was broken after he was arrested and put in the back of a police van earlier this year. His death of course triggering days of protests and civil unrest. Officer William Porter is charged with manslaughter, among other crimes. He faces up to 25 years in prison if convicted. Another day of weather extremes across the nation. Records are being shattered but in very different ways. On opposite sides of the country, record breaking snow in the west, and record breaking high temperatures here in the east. Well hear more from nbcs dylan dreyer. Reporter this mornings commute in colorado a mess. Hundreds of flights cancelled. The same storm dumped nearly a foot of snow in parts of montana. And in salt lake city, more than 9 inches fell, more than all of last winter. In the east, the other extreme. Today, boston warmer than las vegas. And new york milder than phoenix. Here in central rk, we reached a record high of 68 degrees at 1 00 a. M. And its not just new york city breaking records in the middle of the night. Philadelphia hit 69 degrees. Wilmington, delaware, 68. And providence, rhode island reached 64 by 4 00 a. M. The weather extremes all thanks to a strong el nino. Although the seasonal forecast called for warmer than normal conditions through the winter for the northern u. S. , that doesnt mean on any given day we cant have a cold outbreak and a winter storm. Reporter in vermont, 11 of the states 50 ski resorts are open, a serious economic blow. Christmas is a really important holiday for us and its not looking so good. Reporter but for shoppers, warm temperatures are bringing the savings. At walmart, coats on sale for 80 off. A change in the jetstream will bring cooler temperatures to the northeast this weekend. By next week, the warmth will be back, just in time for the First Official day of winter. Dylan dreyer, nbc news, new york. Still ahead tonight, high tech hope for those diagnosed with brain cancer. The experimental device showing great promise that could give these patients something that want so badly more time. Also celebrities swear by it on tv but now hundreds say a popular hair care line has some terrible side effects. Side Technology Empowers us to achieve more. It pushes us to go further. Special olympics has almost five million athletes in 170 countries. The microsoft cloud allows us to immediately be able to access information, wherever we are. Information for an athletes medical care, or information to track their personal best. With microsoft cloud, we save millions of man hours, and thats time that we can invest in our athletes and changing the world. Thwith aches, chills,g. And fever, theres no such thing as a little flu. And it needs a big solution an antiviral. So when the flu hits, call your doctor right away and up the ante with antiviral tamiflu. Prescription tamiflu is an antiviral that attacks the flu virus at its source and helps stop it from spreading in the body. Tamiflu is fda approved to treat the flu in people two weeks of age and older whose flu symptoms started within the last two days. Before taking tamiflu, tell your doctor if youre pregnant, nursing, have Serious Health conditions, or take other medicines. If you develop an allergic reaction, a severe rash, or signs of unusual behavior, stop taking tamiflu and call your doctor immediately. Children and adolescents in particular may be at an increased risk of seizures, confusion, or abnormal behavior. The most common side effects are mild to moderate nausea and vomiting. Antiflu . Go antiviral with tamiflu. Im gellin and zinfandellin. And so is my new bride, helen mcmellin im so happy my eyes are wellin dr. Scholls massaging gel insoles are so soft they make your feet feel outrageously comfortable. Im gellin youre so not gellin dr. Scholls diis critical for brain health . N brain food, hmmm. Ensure has b vitamins that help support brain health now thats smart nutrition. Ensures complete balanced nutrition has 26 vitamins and minerals and 9 grams of protein. Ensure. Take life in. Nobodys hurt, but there will you totastill be pain. New car. It comes when your Insurance Company says theyll only pay three quarters of what it takes to replace it. What are you supposed to do . Drive three quarters of a car . Now if you had Liberty Mutual new car replacement, youd get your whole car back. I guess they dont want you driving around on three wheels. Smart. With Liberty Mutual new car replacement, well replace the full value of your car. See Car Insurance in a whole new light. Liberty mutual insurance. Were back with a promising advance in the battle against cancer. A device that uses electric waves to target the most aggressive type of brain cancer. Tonight, we meet a woman who hopes it can buy her more time with the people she loves. Nbcs Janet Shamlian has the first part of our special series of reports target cancer chasing a cure. Reporter amy has gotten used to her new headgear which she sometimes covers with a hat or wig. Diagnosed with glioblastoma, shes trying a new treatment that a study out today finds is extending the lives of patients who have the deadliest form of brain cancer. It sends electric waves to the brain, preventing cancer cells from multiplying. Its worn almost round the clock and is used with surgery, radiation, and chemotherapy. It cant cure patients like amy. But the study shows it can add five precious months to their lives. This is the first Clinical Trial in a decade to lengthen the lives of patients with glioblastoma. So i think that this is going to be really exciting. Reporter glioblastoma is the most common form of brain cancer. Doctors hope the device will extend the prognosis. We do expect now that it has been fda approved that the use will be more frequent and more widespread. This is always connected to the wires. Reporter amy has been wearing the cap and battery pack since october. Its doing its job while the 41yearold does hers. You did your check, awesome. Reporter teaching math to middle schoolers in rochester, new york. I feel truly blessed that technology and treatments are growing every day, and that i have an opportunity to beat cancer. Reporter the device is expensive, as much as 20,000 a month. And yet priceless. Prolonging life. The mri. Reporter after a devastating diagnosis. Janet shamlian, nbc news, new york. Tomorrow in our series we go inside los alamos to see how the Top Secret Nuclear weapons lab came to the forefront of groundbreaking research that could change the fight against cancer. Were back in a moment with a state of emergency declared in one American City over a toxin. Declared in one American City over a toxin. 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