As we come on the air abc news has learned that the fbi is on the move, investigating evidence that trained terrorists were able to come into the u.S. As refugees and slip right into the heartland. It seems they sneaked in with thousands and thousands of legitimate refugees from iraq and there are new images tonight showing why the fbi is so concerned. Brian ross starts us off. Reporter: This fbi surveillance video was made in kentucky. It shows an admitted al qaeda terrorists who had already killed american soldiers in iraq trying to get weapons to kill more of them. Authorities tell abc news he may be just one of dozens of men with american blood on their hands, who were mistakenly allowed to settle in the u.S. As refugees. These are trained terrorists in the art of bomb making that are inside the united states and quite frankly from a homeland security perspective, that greatly concerns me. Reporter: The kentucky case unfolded in the city of bowling green where two iraq al qaeda cell members had moved into quiet neighborhoods living here and author. Authorities say that waad alwan and ha maddy were able to come to the u.S. The system failed here. If you are asking my opinion I would say the system failed. Reporter: In iraq the two men were part of an al qaeda group that carried out relentless series of attacks against u.S. Forces including one that killed four members of the pennsylvania national guard in 2005. These two individuals are evil. Reporter: During the undercover operation the men said they wanted to carry out new attacks in the u.S. And kill an army captain who had served in iraq. They wanted to assassinate this particular u.S. Captain. Reporter: A big break in the case came out of this fbi warehouse in washington. This is basically america's bomb library. Reporter: The repository for some 100,000 bombs used against american targets around the world. Fbi technicians were able to find the fingerprints of one of the kentucky men on a bomb that had been planted in iraq in 2005. You can see right here -- and right there. Yep. What was that like when you made the match? Like finding a needle in a haystack. Reporter: There is an urgent effort to go through a huge backlog of other bombs looking for fingerprint matches with other refugees in the u.S. Who also may have been able to hide their al qaeda ties. We are currently supporting dozens of current counter terrorism investigation like that. Where you're looking for prints of people who are in this country now? That's correct. Reporter: The two kentucky men both pleaded guilty and are now in prison but the discovery led to a six month suspension of the refugee program which has resettled tens of thousands of legitimate refugees. The service says it has tightened security and background checks and hoping in effect they have closed the barn door in time to keep other terrorists out. They're on it. 700 agents assigned to the task. You'll have more tonight, a full report on "nightline" as well.