Washington ? Federal investigators have documented 1,300 cases of lost, stolen or abandoned radioactive material inside the United States over the past five years and have concluded there is a significant risk that terrorists could cobble enough together for a dirty bomb.

Studies by the Energy Department’s Los Alamos laboratory and the General Accounting Office found significant holes in the nation’s security net that could take years to close, even after improvements by regulators since Sept. 11, 2001.

“The world of radiological sources developed prior to recent concerns about terrorism, and many of the sources are either unsecured or provided, at best, with an industrial level of security,” the Los Alamos lab concluded two months ago in a report that was reviewed by The Associated Press.

The report concludes that the threat of a so-called dirty bomb that could disperse radiological materials across a wide area “appears to be very significant, and there is no shortage of radioactive materials that could be used.”

Security improvements under way “are unlikely to significantly alter the global risk picture for a few years,” it added.

The FBI repeatedly has warned law enforcement over the past year that al-Qaida was interested in obtaining radiological materials and creating a dispersal bomb. The most recent warning came after authorities received an uncorroborated report a few weeks ago that al-Qaida might be seeking material from a Canadian source.

Nuclear Regulatory Commission spokeswoman Beth Hayden said the agency recognizes the potential dangers of such materials and al-Qaida’s interest in them — “there are millions of sources,” she said.

But she added that most of the 1,300 lost radiological sources were subsequently recovered and the public should keep the threat in perspective.

“The ones that have been lost and not recovered, I’m told, if you put them all together, it would not add up to one highly radioactive source,” Hayden said. “These are low-level sources.”

The Los Alamos analysis specifically cited concerns about the transportation of large shipments of radioactive cobalt from industrial sites, as well as lax security at hospitals that use radiological devices to treat and diagnose patients.