"The fact of the matter is that in Iraq there have been improvements made, and successive investigations have seen that improvements were made, and they were not brushed off," Rumsfeld said.

The Pew Research Center released results of a poll that it said showed the prison scandal "registered powerfully with the public — fully 76 percent say they have seen pictures depicting mistreatment of Iraqis by U.S. soldiers."

The poll results also found for the first time a majority of Americans, 51 percent, said the war in Iraq was not going well, the center reported.

Both the Republican and Democratic presidential campaigns attacked each other Wednesday over Iraq.

Sen. John Kerry, D-Mass., his party's presumptive presidential nominee, told Associated Press Radio that the Bush administration had been guilty of "one miscalculation after another, frankly. And arrogance that has lost America respect and influence in the world."

Steve Schmidt, spokesman for Bush-Cheney '04, said in a statement that Kerry had "stooped to a new low" in his attacks. "John Kerry never misses an opportunity to play politics with the war on terror, and his appalling comments call into question his ability to lead America during these challenging times," Schmidt said.

In the case of Berg, whose death was videotaped and shown on the Internet, further details emerged but much remained unclear.

The 26-year-old Pennsylvania native traveled independently to Iraq and tried to drum up work repairing cellular telephone towers. He was detained in late March, and the FBI released a statement Wednesday that agents had interviewed him in Mosul while he was in the custody of Iraqi police.

The FBI said it checked records and reported that it had "no derogatory information . . . that would warrant further detention." The FBI said its agents and Coalition Provisional Authority officials told him that Iraq was dangerous and offered to help him secure safe passage out of the country. "Mr. Berg refused these offers. . . . He also refused government offers to advise his family and friends of his status," the statement said.

Berg's parents have said he was in U.S. custody, although officials in Baghdad flatly denied that Wednesday. Berg was released in early April, a day after his parents filed a federal lawsuit claiming he was being wrongfully held by U.S. authorities.

"Nicholas Berg was an innocent civilian who was in Iraq to help build a free Iraq," Bush said. "There is no justification for the brutal execution of Nicholas Berg, no justification whatsoever."

Islamic militants who claimed a link to Abu Musab al-Zarqawi, an associate of al-Qaida leader Osama bin Laden, posted the chilling video on a Web site and warned that more such killings will follow.