A suicide bomber blew himself up at a Baghdad police academy on Tuesday, sending terrified survivors rushing for the shelter of concrete blast walls where a second bomber detonated explosives strapped to his body, witnesses said.

The double suicide attack -- the latest, minor tactical variation in the daily bombings that over nearly three years of war have reduced much of Baghdad's public areas to bleak gray blast walls and rubble -- killed at least 27 people, witnesses and U.S. and Iraqi authorities said. Police quoted by news agencies reported a death toll as high as 34, one of the deadliest attacks on Iraqi security forces in months.

The attack targeted Baghdad's main police academy, and the majority of the dead were recruits. An American contractor was among roughly 50 wounded, the U.S. military said.

One witness reported that some of the victims were trampled as survivors squeezed through a narrow opening between blast walls.

Police Capt. Salah Hassan Falahi, interviewed by phone after witnessing the attack, said the assailants hit as police officers and recruits were assembling for what he said was the academy's standard roll call before lunch each day. The bombers slipped in as the last of them were lining up, he said.

"When the explosion took place, there was chaos and panic," Falahi said.

Falahi and a second person at the scene, who did not directly witness the first blast, reported the panicked rush to the blast walls. The second suicide bomber, they said, ran alongside survivors, setting off his charge just as those around him thought they had reached safety.

Witnesses said U.S. forces detained Iraqi security workers at the entrances to the academy to question them about how the bombers, who wore vests packed with explosives, entered the heavily guarded grounds.

In other violence, Iraqi soldiers patrolling a highway linking Baghdad with Jordan found the bodies of 11 men by the side of the road, handcuffed and shot in the head, Lt. Hussein Hadhood said. The men were in civilian trousers and shirts. The bodies, found Monday, had been partially eaten by animals, leading police to believe they had been there for several days, Hadhood said.

There was no immediate word on the identities of the men. Sunni Arabs, a minority in Iraq, repeatedly have accused militias linked to the Shiite Muslim-led government of operating death squads that target Sunnis. Hundreds of bodies of Sunni men have been found in similar discoveries in recent months along remote stretches of road.

Iraqi police found the bodies of another nine men by the side of a road between the predominantly Sunni cities of Fallujah and Ramadi, according to an Iraqi police captain who would not give his name. Police believe the victims were set upon by armed men -- possibly bandits -- who stole their car, the captain said.

Highway robberies, kidnappings and other crimes have soared since the toppling of Saddam Hussein's dictatorship in 2003, making travel between regions of the country dangerous for foreigners and Iraqis. Kidnapping of Westerners has rebounded in recent weeks after months of relative decline.

Two Arabic-language television networks aired video Tuesday of a man described as a kidnapped American contractor. The man, who had bright blond hair, was shown seated in a chair with both arms behind his back, as if tied.

Other footage showed what appeared to be an American passport. Relatives of the man identified on the passport could not immediately be reached for comment. An insurgent group asserted responsibility for the kidnapping. Al-Jazeera, one of the networks that broadcast the images, said it could not confirm the veracity of the claim.

U.S. authorities said they were investigating.

Another U.S. citizen was among four peace activists taken hostage Nov. 26 by a group calling itself the Swords of Righteousness Brigade. Two Canadians and a Briton, members of Christian Peacemaker Teams, an antiwar group based in Chicago, were seized with him.

A French engineer was taken hostage in Baghdad on Monday, and a German archaeologist was abducted near Mosul on Nov. 26.

On Tuesday, President Bush said the United States would work for the return of captive Americans in Iraq. "We, of course, don't pay ransom for any hostages," Bush told reporters at the White House. "What we will do, of course, is use our intelligence-gathering to see if we can't help locate them."

The violence and kidnappings have taken place in a capital decorated with white election banners for parliamentary elections Dec. 15. One of the most prominent candidates, former prime minister Ayad Allawi, came under continued political attack Tuesday.

Ahmed Chalabi, a rival of Allawi who was once a U.S. favorite to lead Iraq after the war, held a news conference in Baghdad to elaborate on news media and U.S. reports of corruption in Allawi's interim administration, which was put in place by the United States and United Nations in June 2004, and ended in February. Chalabi displayed documents that he said showed Allawi had signed more than $1 billion in defense contracts for helicopters and vehicles that were never delivered.

"I found it my responsibility to tell people some of the details of corruption that spread in Iraq," said Chalabi, himself a candidate in the coming elections, who was convicted in absentia by a Jordanian court in 1992 of embezzlement and bank fraud charges and was sentenced to 22 years in prison. Chalabi says the prosecution was politically motivated. "The Iraqis shouldn't go to ballot boxes before they know the truth," he said about his rival.

In Najaf, about 90 miles south of Baghdad, rocket-propelled grenades hit an office of Allawi's National Accord party Tuesday evening as party officials met inside, said Abdulal Wahid Esawi, a National Accord leader in the Shiite holy city. There was one injury, Esawi said.

The attack came two days after a mob attacked Allawi, forcing his convoy to hurriedly flee Najaf.

Ibrahim Janabi, an aide to Allawi, said the attacks were political, targeting the secular Shiite, partly for his leadership and partly for his opposition to a Shiite-backed plan to decentralize the national government and create a Shiite state in southern Iraq.

"They know well how strong Dr. Allawi is," Janabi said.

Special correspondents Omar Fekeiki in Baghdad and Saad Sarhan in Najaf contributed to this report.

A wounded Iraqi police cadet describes the attacks on Baghdad's main police academy that injured about 50, including an American contractor.