While U.S. forces search for the "smoking gun" to prove that Saddam Hussein was hiding weapons of mass destruction, nuclear facilities in Iraq have been left unchecked and countless amounts of radioactive materials such as uranium (search) have gone missing, critics charge.

The Tuwaitha (search) nuclear complex, 20 miles south of Baghdad, is just one of several sites that has reportedly experienced massive looting, and is now host to the only United Nations inspections team that has been allowed into Iraq since March.

Speaking on condition of anonymity, diplomats said late Friday that the inspection team from the International Atomic Energy Agency (search) has been able to account for most of the uranium looted from Tuwaitha, but the scope of its two-week-long mission to assess the inventory was limited to natural and low-enriched, or "yellowcake" uranium, not hundreds of other radioactive materials stored at the facility.

Nor was the team, which was restricted by the U.S. government, able to test Iraqis in the nearby community for radioactive exposure, said the diplomats, who are familiar with the workings of the IAEA.

Calls to the IAEA offices in New York were not returned, and U.S. Central Command (search) officials could not confirm the diplomats’ assessment on Saturday.

Experts say Tuwaitha, which was sealed off by U.N. inspectors in 1991, holds 500 metric tons of uranium and other radioactive materials that could pose health risks or even a terrorist threat if it gets into the wrong hands.

It is one of approximately 1,000 known nuclear facilities in Iraq, several of which have reportedly been looted for their U.N.-prohibited materials in recent months.

"I'm confident that we did not plan adequately for the security of those sites," said Robert J. Einhorn, a former State Department official and senior adviser at the Center for Strategic and International Studies (search), a Washington-based think tank.

"It was not a sufficiently high priority for us."

But just how much uranium and other radioactive material has actually been stolen, and who may now have access to the harmful substances, remains unknown.

After several requests, the IAEA team was allowed into the country in early June by the U.S. government to assess the damage at the Tuwaitha complex, which was abandoned by Iraqi army forces on or about March 10, according to Pentagon officials.

Witnesses in the area said looters breached the facility shortly after it was abandoned and stole low-grade enriched uranium and other radioactive materials that could be used to make a "dirty bomb," according to Andy Oppenheimer, a London-based correspondent for Jane's Information Group's Terrorism and Security Monitor.

U.S. officials downplayed the impact of the incident. In a briefing on June 5, they said the site, which is located on a massive 23,000-acre campus that also includes the Baghdad Nuclear Facility, was indeed breached by looters, but has been secured by U.S. troops since April 7.

Officials said that along with physical damage to the site and its contents, radioactive material — including the yellowcake second-stage processed uranium held at the facility — was found on the ground, inside and outside, when U.S. troops arrived.

More recently, in a briefing before the House Armed Services Committee on June 18, Gen. Peter Pace, vice chairman of the Joint Chiefs of Staff, assured the committee that "the vast majority of what was there is still there and accounted for."

He said coalition forces with the IAEA team reported that 20 percent of the barrels containing the yellowcake were indeed missing, but believed that most of the uranium had been accounted for. Officials have also reported recovering 100 barrels from the local population that look like the ones used to contain the yellowcake at the facility.

They also said they recovered "five radioactive sources and other items," but did not elaborate.

Diplomats confirmed Friday that the team believed that most of the missing uranium was found on the site, dumped out by looters who appeared to have wanted only the containers.

But depending on the levels of radiation and exposure, such material could pose a significant health risk if any of it had been ingested or dispersed into the air, said experts.

Citing local sources, Oppenheimer insists that uranium was either stolen and sold or tipped out, and containers are now being used for domestic purposes — like milking cows and storing drinking water — in the nearby community.

He said public health officials say they are recognizing symptoms they believe are related to radioactive exposure — though there has been no independent verification of how widespread the exposure, if any, has occurred among the people there.

"The local doctors have been seeing people with all the signs of radiation sickness, but very little has been reported since," said Oppenheimer. Pentagon officials said they have been conducting weekly meetings with members of the Iraqi Atomic Energy Commission, coalition forces and U.S. health experts. They are assessing what risks the local population and U.S. troops might be exposed to at Tuwaitha, and will soon conduct "a wider search and a health risk assessment of the surrounding civilian area," said a senior military official.

According to experts, while the yellowcake's relatively low radioactivity would not make an attractive source for a dirty bomb, it wouldn't be difficult to build such a weapon with other radioactive materials found at the site, like cobalt isotopes, plutonium and cesium. While a dirty bomb is not nuclear, it can disperse varying amounts of radiation into the air and cause great harm and panic, they said.

"Any amount of that material in any conversion can be used to cause panic and would be very expensive to clean up," said Oppenheimer. If there were restrictions on the IAEA mission at Tuwaitha, there is no telling how much of those materials were stolen before U.S. troops arrived, nor would the inspections address reports of looting at several other nuclear sites.

But Einhorn cautioned against what he believes to be "wildly exaggerated reports."

"There has been a lot of hype," he added. "My guess is there are materials that have gone out into the public that are a health hazard. I don't know how serious a health hazard, but in terms of the material that can make bombs, the experts need to inventory and see what it is missing."

Back in Washington, lawmakers are hesitant to read too much into the situation, and prefer to wait until IAEA officially announces its findings at Tuwaitha.

Harold Stevanas, spokesman for the House Armed Services Committee, said the issue was briefly discussed in the June 18 hearing.

"Obviously it is a concern, but we're waiting for more information," said Stevanas. "It's too early to draw conclusions."

The Associated Press contributed to this report.