Despite tightened security, terrorists still have a "very significant" chance of obtaining enough radioactive ingredients to create a dirty bomb (search), federal investigators conclude.

This finding arises from studies that document more than 1,300 disappearances of radioactive materials in the United States over the past five years. Most have been recovered, but some losses remain unsolved.

"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," a report written by the Energy Department (search)'s Los Alamos (search) lab concluded.

That study and three others by the General Accounting Office, reviewed by The Associated Press, cited significant holes in the nation's security net that could take years to close, even after improvements by regulators since Sept. 11, 2001.

The Los Alamos report concludes that the threat of a so-called dirty bomb that could disperse radioactive debris 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 Qaeda was interested in obtaining radiological materials and creating a dispersal bomb, most recently after authorities received an uncorroborated report a few weeks ago that Al Qaeda 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 Qaeda's interest in them. "There are millions of sources," she said. But she added most of the 1,300 lost radiological materials 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 top Democrat on the House Homeland Security Committee says the studies show security efforts fall short of the need.

"Even though for years we have known of the threat that terrorists would use 'dirty bombs' to attack the United States, I am alarmed at the government's inadequate response to this very real threat. The economic and health costs of such an event would be staggering. It appears we don't even know how much material exists that could be used for such weapons or even where it is being kept," Rep. Jim Turner, D-Texas, said.

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.

The GAO, Congress' investigative arm, detailed how terrorists could abuse the legal method for obtaining radiological sources because the NRC takes as long as a year to inspect facilities after it mails them a license for such materials. "It is possible that sealed sources can be obtained for malicious intent," the GAO told the Senate recently.

NRC Commissioner Edward McGaffigan Jr. said the GAO concerns were overstated, focusing on materials with extremely low radioactivity. He said his agency has been taking steps for months to more securely ship and store high-risk sources.

"We honestly think we are doing a very aggressive and excellent job in this area, but we have obviously more to do," McGaffigan said in an interview. "Our view is we don't want to lose any of them, and we are going to have cradle-to-grave controls as soon as we possibly can for high-risk sources."

The government is undertaking its first-ever inventory of who possesses radioactive materials and how much they possess, he said.

The GAO questioned whether the NRC has moved fast enough to secure sealed sources — devices that contain small amounts of radiological materials used in construction and hospitals.

"The number of sealed sources in the United States is unknown because NRC and states track numbers of licensees instead of sealed sources," the GAO told the Senate in August.

Two universities told the GAO about cases in which doors to rooms with nuclear materials had been found unlocked or open.

The congressional investigators found that many of the 114 universities that possess radioactive plutonium-239 have tried unsuccessfully to return it to the government. The Energy Department doesn't have enough secure storage space, the investigators said.

The congressional investigation for the first time tallied the number of times sealed radiological materials have been lost, misplaced or stolen. They found more than 1,300 instances inside the United States since 1998. While most have been recovered, the report cited a handful of harrowing, unsolved losses.

In March 1999, an industrial radiography camera containing iridium-192 was stolen from a Florida home. The camera has not been recovered despite an FBI investigation. The NRC believes the material should have degraded by now and no longer be useful for a bomb.

A North Carolina hospital discovered in March 1998 that 19 sealed sources of radiological material, including the highly dispersible cesium-137, were missing from a locked safe. They have not been found.

Security improvements are being made. For instance, the NRC requires tighter security by companies that use soil analysis gauges that contain radiological materials. There are some 20,000 of them used nationwide by more than 5,100 licensees. The devices are lost or stolen at a rate of one a week, officials said.

The GAO and Los Alamos security reviews made several recommendations. They include keeping licensed sources from getting radiological materials until after they are inspected, improving structural security at high-risk locations and toughening federal, state and international regulatory controls.

"These efforts are unlikely to significantly alter the global risk picture for a few years, although the risks regarding certain sources and circumstances could change more quickly," the Los Alamos study conceded.