U.S. defense officials say they found more radioactive material at a looted Iraqi nuclear site than they expected, raising questions about the reliability of prewar intelligence about the site.

The officials didn't say how much material they expected to be at the Tuwaitha (search) site or how much more they found.

It's unclear whether the discovery means the Americans' information was wrong or the Iraqis had moved material to the site before the war, said three top military and defense officials who briefed reporters on condition of anonymity.

They said they could not determine if any radioactive material had been stolen from the storage site about 30 miles southeast of Baghdad (search). Earlier, U.S. officials in Iraq said at least 20 percent of the site's tons of radioactive material was missing.

American forces have collected more than 100 empty metal barrels and five radiological devices by paying $3 bounties for items suspected of having been looted from Tuwaitha, the defense officials said Thursday.

A visit this week by the International Atomic Energy Agency (search) could help clear up the confusion.

The IAEA gathered the radioactive material and sealed it at the Tuwaitha storage site after the 1991 Gulf War and has inspected the facility once a year since. A team of seven IAEA experts is scheduled to begin an assessment at Tuwaitha on Saturday.

The United States had resisted allowing the IAEA officials back into Tuwaitha, which had been the centerpiece of Iraq's nuclear program.

Even now, Pentagon officials stress that the IAEA visit is a one-time event to enforce the nuclear Nonproliferation Treaty, not a weapons inspection that might set a precedent for U.N. searches for weapons of mass destruction in Iraq.

American troops and weapons experts will accompany the IAEA officials wherever they go, an arrangement the Pentagon officials said was for safety.

Before the war, IAEA inspectors concluded Iraq didn't have an active nuclear weapons program -- a finding U.S. leaders blame in part for their failure to win broader international support for the war.

The tensions with the IAEA come amid persistent questions about the U.S. hunt for evidence of the nuclear, biological and chemical weapons programs President Bush said he went to war to eliminate. No such weapons of mass destruction have been found, although Bush says the discovery of two trailers crammed with germ-growing equipment proves Iraq had a biological weapons program.

American forces have visited all but a handful of the 20 to 30 other storage sites for radioactive materials in Iraq, but have no plans to allow the IAEA to visit them, U.S. officials said.

Tuwaitha clearly had been picked over by thieves. The fence and 12-foot concrete wall around the three storage buildings for radioactive material had huge gaps, and U.S. Marines found the main gate open when they arrived April 7.

Inside, some radioactive material had been scattered. Radioactivity measurements inside the three buildings found levels two to 10 times background levels, a senior U.S. military official in Baghdad said, joining the news briefing via a satellite link.

Local Iraqis have told the Americans that Iraqi soldiers guarding Tuwaitha left on March 10, before the war started, and civilian guards abandoned the site March 20, the day before American ground forces entered Iraq from Kuwait.

Although reporters saw looters inside the radioactive material storage site after the Americans arrived, the defense officials said Thursday they had no evidence of any looting there after April 7. Other looters have been captured elsewhere on the sprawling, 23,000-acre Tuwaitha site, however, they said.

The looting has raised the possibility that terrorist groups could have obtained material for a radiological "dirty bomb" from the site. None of the material at Tuwaitha was of high enough quality to make a nuclear bomb.

Most of the uranium stored at the site is "yellow cake," a slightly processed form of uranium ore the color and consistency of yellow corn meal. Some low-enriched uranium also is stored at the site -- uranium processed to enhance the percentage of the element's isotope that is most useful for nuclear reactor fuel and weapons. That material is not the highly enriched uranium needed for a nuclear bomb.