The senior safety officer of the U.S. government's nuclear weapons laboratory in Los Alamos, N.M., says the facility is not safe.
"I am the senior guy that's supposed to be able to say whether or not Los Alamos is safe or not, and right now, I can't say that it's safe," Christopher Steele admits.
According to Steele, the equivalent of 5,000 pounds of plutonium in barrels of radioactive waste is stored outside the laboratory under a tent. There it is vulnerable to theft and would be extremely dangerous if detonated on site, where it would release a radioactive plume.
"The magnitude of this sort of catastrophe, it's hard to fathom," Steele says.
According to the official who oversees the nation's nuclear weapons facilities, security at Los Alamos has improved since 9/11.
"We're not going to let people get anywhere near nuclear material," says Linton Brooks, administrator of the National Nuclear Security Administration. "If they come, they're going to die en route."
In security tests at nuclear facilities since 9/11, however, so-called "black hat" attack teams have found continued failures.
Congressional investigators also say Los Alamos and other U.S. nuclear facilities have still not created a good defense against suicide attacks.
"If a terrorist is willing to be blown up in the process -- and they are -- we have tremendous vulnerability," claims Rep. Christopher Shays, R-Conn., who is the chairman of the House Government Reform Subcommittee, which oversees all matters pertaining to national security, including efforts to combat terrorism.
Similar concerns surround the extensive transport of nuclear materials and radioactive waste along the nation's highways.
"Explosive material plus radioactive material equals dirty bomb," says Graham Allison, a Harvard University professor and director of the Belfer Center for Science and International Affairs.
Although drivers are supposedly trained to spot suspicious activity, an ABC News crew trailed a truck convoy for 15 hours and 850 miles while in the same car and was never challenged.
"The element of surprise is going to beat them in a very short time," Ronald Timm, a former Department of Energy security analyst, said of an attack on a convoy transporting nuclear materials. "You'd own the load in 60 to 120 seconds."
Gaping holes in security were also found during an ABC News investigation at many of the little-known nuclear research reactors operating on 25 college campuses.
Located in the middle of crowded college campuses, the reactors, which run on radioactive fuel, were inadequately protected by unmanned guard booths, unlocked doors, and access gained without background checks or metal detectors.
ABC News' Vic Walter and Avni Patel contributed to this report.