sold through the notorious fast and furious operation, ignited a firestorm. it is unknown if the weapon that killed ivy has any link to that now-famous controversy. what is clear here is just how dangerous life can be. >> agent ivy died in the line of duty, protecting our nation against those who threaten our way of life. his death only strengthens our resolve. >> reporter: the terrain out here is so rough it could be days before agents are done processing this crime scene. for now the flags here at this station remain lowered and the agents here remain in mourning. cecilia vega, abc news, naco, arizona. >> our thanks to cecilia. president obama and mitt romney have been mostly behind closed doors ahead of their debate tonight in denver. that's left their running mates to generate some fireworks, so to speak, on the campaign trail. vice president joe biden got things started with this comment about romney's economic plans. >> how they can justify raising