U.S. and Iraqi officials have confirmed the theft of at least 6,000 artifacts from Iraq's National Museum of Antiquities during a prolonged looting spree as U.S. forces entered Baghdad two months ago, a leading archaeologist said yesterday.

University of Chicago archaeologist McGuire Gibson said the U.S. Bureau of Immigration and Customs Enforcement told him June 13 that the official count of missing items had reached 6,000 and was climbing as museum and Customs investigators proceeded with an inventory of three looted storerooms.

The June 13 total was double the number of stolen items reported by Customs a week earlier, and Gibson suggested the final tally could be "far, far worse." Customs could not immediately obtain an updated report, a spokesman said.

The mid-June count was the latest in a confusing chain of seemingly contradictory estimates of losses at the museum, the principal repository of artifacts from thousands of Iraqi archaeological sites documenting human history from the dawn of civilization 7,000 years ago to the pinnacle of medieval Islam.

It now appears, however, that although the losses were not nearly as grave as early reports indicated, they go far beyond the 33 items known to have been taken from the museum's display halls. Gibson said looters sacked two ground-floor storerooms and broke into a third in the basement. Two other storerooms appear to have been untouched.

Gibson noted that there are "thousands of things that are broken" but not listed as missing. And teams of archaeologists sent by the National Geographic Society found widespread looting of artifacts from sites outside Baghdad. None of these are museum pieces, and most were simply plucked from the ground.

"Like any museum, the display collection is an iceberg," Gibson said. "Because this is an archaeological museum, there's a huge amount of stuff that's important to the sites themselves and to researchers, but never goes on display."

Looters broke into the downtown Baghdad museum and sacked it for several days in early April as U.S. forces toppled the government of Saddam Hussein and took possession of the Iraqi capital. U.S. soldiers were harshly criticized for standing idle as the looters rampaged through the building.

The museum housed 170,000 numbered items and thousands more artifacts that had either not yet been catalogued or had been set aside in a ground-floor "study collection" storeroom for researchers to examine.

Reporters and investigators arriving in the first days after the looting saw a virtually empty museum that had been thoroughly trashed. They assumed the worst, Gibson said, an impression that the museum staff did not seek to dispel.

In fact, the staff -- anticipating possible looting -- had spirited away a huge portion of the inventory, including almost everything portable in the display collection, and stashed it either in the basement or in off-site bunkers, Gibson said. Staff had also hidden a gold collection in a Central Bank vault during the 1991 Persian Gulf War and never removed it.

When U.S. authorities took their first close look at the damage, it appeared the finest artifacts had been "cherry-picked" by thieves with inside knowledge. Some U.S. officials suggested that staff members might have been complicit.

"This was unfortunate" but easily explained, Gibson said. Bitterly offended by U.S. forces' failure to protect the museum from the looters, staffers "were not going to give information on where things were," he added. Today, museum staff and U.S. investigators from Customs and the FBI have "a very good relationship," Gibson said.

Although the display collection lost only a few heavy, nonremovable artifacts that were either cut in pieces or ripped from their pedestals, the overall toll was much worse. Staff members began to inventory the museum's five storerooms in May. Losses there numbered in the thousands.

Both ground-floor storerooms had been looted, Gibson said. One housed the study collection, while the other held shelved artifacts and about 10 steel trunks containing as-yet unnumbered material from recent digs. All the trunks had been opened and emptied, Gibson said.

Two basement storerooms appeared to be untouched, including one containing most of the museum's priceless collection of cuneiform tablets, Gibson said. The third had been breached, however, and contained "some of the most important stuff in the museum, including pottery and ivory inlays," he added.