A major radioactive waste site in Iraq has been looted and U.S. officials have little idea whether nuclear material is missing, according to a news report.

A report in Sunday's Washington Post says a specially trained Defense Department team finally was dispatched to the Baghdad Nuclear Research Facility (search) after a month of flip-flopping over whether to survey the site.

It's feared that dangerous material may be sold on the black market and could end up in the hands of terrorist groups.

Saturday's finding was the second discovery since the end of major fighting in Operation Iraqi Freedom (search) in which a known nuclear cache was looted so much that authorities couldn't even tell if -- or what -- materials were missing.

"I know we have a number of teams that are interagency teams" who are searching suspect sites, Defense Secretary Donald Rumsfeld (search) told reporters Sunday.

"When there's a war, people go in an area and take things, we've seen that with football games and soccer games in other countries," Rumsfeld said. "It's a natural, apparently, a not infrequent behavior patterns for human beings. It's unfortunate."

Rumsfeld said first reports -- which he noted were frequently wrong -- say items were in fact taken from the nuclear site but "what they may have taken I don't know."

Seven sites related to Iraq's nuclear program have been visited by the Pentagon's "special nuclear programs" teams since the war winded down, the Post reported.

The Baghdad Nuclear Research Facility contains the remains of reactors bombed by Israel in 1981 and the United States during the first Gulf War in 1991. It has stored industrial and medical wastes, as well as spent reactor fuel. High-energy isotopes at the site, including cesium and cobalt, have been sought by terrorists hoping to use conventional explosives to scatter radioactive particles.

In other words, people who get hold of the material could make a "dirty bomb."

The Tuwaitha Nuclear Research Center was also looted.

U.N. weapons inspectors visited the site multiple teams in the months leading up to the war, noting tons of partially enriched uranium and natural uranium -- metals used for making the core of a nuclear weapon. Iraqis took off with computers, furniture and equipment. It's not yet known if dangerous material was stolen.

U.S. officials don't know exactly what is missing because of the infighting going on between the White House and the Vienna-based International Atomic Energy Agency, nuclear weapons experts who have searched in Iraq. The Bush administration is determining how much to involve the IAEA in its current search for weapons.

U.S. forces haven't yet been able to enter Tuwaitha's nuclear storage areas, but a short inspection on April 10 found that one door to the site was breached.

On April 10, a smaller U.S. team saw reported damage to the site and high levels of radioactivity, according to the Post. They recommended an increase in security. On April 11, the IAEA listed the Baghdad Nuclear Research Facility and Tuwaitha as the two in most dire need of protection.

Mohamed ElBaradei, director-general of the IAEA, said he received assurances from Washington that the Tuwaitha center would be protected and that access to the complex would be restricted.

The Army's 3rd Infantry Division was sent to control the gate.

"I don't believe this," one Special Forces soldier said this weekend. "They let workers in here for the past week," referring to local Iraqi workers.

Looting has been rampant in Saddam's palaces and other places. U.S. military forces are working with Iraqi police to bring order to various parts of the country.

But as many as 40 looters a day ravaged the Baghdad nuclear site, the Post said. U.S. soldiers detained 62 of them on Friday.

"Looters, they see us in Bradleys or on foot," said Capt. Blaine Kusterle. "They can outrun us easily because they have a 300-meter start."

Among the items seized from looters was a Braun sedimenter, an autoclave, a Nikon photo microscope, toxic gas monitors, and a machine to measure tiny particles with laser diffractions.

The first sign that dangerous isotopes may be afoot came when a monitor in the rubble began beeping, the Post said. U.S. soldiers found a yellow crate bearing the warning, "CAUTION RADIOACTIVE MATERIAL." It was determined that the box was emitting gamma rays but nothing too harmful.

Nuclear specialists also found what is believed to be a permanent storage site for low-level nuclear waste.

They also stumbled upon a black corrugated metal shed next to a low stone storage area, a site that U.N. inspectors know as Building 55. The IAEA lists those structures as "mechanical workshops and stores."

An Army Special Forces captain said he got a "huge spike" on his detector from 15 feet away indicating something powerful was in a metal storage cylinder the size of a small fire hydrant, the Post said. And there were more cylinders, all corroding. The lock on the shed's door was broken.

"Everybody who was inside that place, just go and stand over there," said Navy Cmdr. David Beckett, who directs special nuclear programs at the Defense Threat Reduction Agency. Everyone was decontaminated.

Nearby were 19 small yellow cylinders and four large gray ones that were giving off so much gamma and neutron radiation that the U.S. team couldn't determine the results, the Post reported.

"There are many radioactive areas within the berm … clearly, they do not appear to be adequately protected," David Albright, an expert on the Iraqi nuclear program, told the Post. "If any radioactive material has been taken, it could pose a significant risk to those who have it. Does the military appreciate the risk?"

Security remains a concern at the Tuwaitha site.

Tuwaitha contains 1.8 tons of low-grade enriched uranium and several tons of natural and depleted uranium. The uranium was inspected by the U.N. nuclear agency twice a year before inspectors were kicked out of Iraq four years ago.

The Tuwaitha complex, run by the Iraqi Atomic Energy Commission, was the heart of Saddam's former nuclear program and was involved in the final design of a nuclear bomb before Iraq's nuclear program was destroyed by U.N. teams after the 1991 war.

In early April, a Marine Corps combat engineering unit claimed they found an underground network of laboratories, warehouses and bombproof offices beneath the 70-building complex.

The Marines said 14 buildings at the site emitted unusually high levels of radiation, and that a search of one building revealed numerous drums containing highly radioactive material.

But the IAEA said Iraq was allowed to keep several tons of low-grade uranium and other nuclear material there under seal because the material couldn't be used directly for weapons.

The Pentagon and State Department are trying to draw up guidelines for a U.S. search team to conduct a survey.

"It's very distressing," a nuclear expert close to ElBaradei told the Post. The agency "expects measures to be taken so that the looting that took place a month ago will not continue to take place this month. This material should not be moved."

Fox News' Liza Porteus contributed to this report.