Seven U.S. Marines were killed on Monday in an apparent suicide attack when a car bomb exploded near their military convoy on the outskirts of Fallujah, the U.S. military reported. The attack, which also killed three Iraqi National Guardsmen, was the deadliest against U.S. troops in four months.

The American casualties were members of the 1st Marine Expeditionary Force, which is responsible for security in Anbar province, a stronghold for Sunni insurgents west of Baghdad. The names of the dead were withheld until their relatives could be notified.

The bomb detonated as the convoy traveled down a barren stretch of road nine miles from Fallujah, U.S. officials said. Two Humvees were reduced to smoldering wreckage, video footage from the Arab satellite channel al-Arabiya showed. U.S. forces removed the bodies and military helicopters flew in.

"This desperate act of inhumanity will only serve to strengthen our commitment to the Iraqi people," the U.S. military said in a statement. "Our forces will continue to stay the course in order to ensure Iraqi security forces have everything necessary to set the conditions required to foster rule of law and revitalization in Iraq."

Marine patrols have not entered Fallujah since the end of a three-week siege in April; the city has been under the control of insurgents. The U.S. military has targeted buildings in the city with periodic airstrikes in an attempt to ferret out the insurgents. U.S. officials say they believe Jordanian guerrilla leader Abu Musab Zarqawi is using the city as his base of operations.

The killing of the seven American troops on Monday represented the highest toll in a single attack since April 29, when eight U.S. soldiers were killed in a car bombing in a southern suburb of Baghdad.

[The U.S. military said Tuesday that one U.S. soldier was killed and another injured late Monday by a roadside bomb near Baghdad, news services reported.]

Meanwhile Monday, the Interior Ministry announced that authorities no longer believe that a man in Iraqi police custody is Izzat Ibrahim Douri, a top deputy to former president Saddam Hussein and one of the U.S. military's two most-wanted fugitives, along with Zarqawi. Ministry spokesman Sabah Kadhim said medical tests concluded that the man was a relative of Douri's. "We are investigating with him to reach the whereabouts" of Douri, Kadhim said.

Iraqi officials said on Sunday that they had captured Douri in a raid on a clinic near Tikrit, Hussein's home town. The report drew widespread coverage on Arab TV channels, as U.S. authorities cast doubt on the claim.

In a separate development Monday, kidnappers released a Turkish truck driver who had been held hostage. The release came after his two employers agreed to demands that they stop working in the country, according to al-Arabiya. The hostage, Midhit Civi, said in a video aired on the station that his captors had treated him well.

U.S.-led forces currently have 5,500 suspected insurgents in custody, Maj. Gen. Geoffrey Miller, deputy commander of detention operations in Iraq, said on Monday. Miller said interrogators were having increasing success picking up useful intelligence using new questioning procedures introduced at the two main U.S. detention facilities in Iraq following revelation of prisoner abuse by U.S. soldiers.

The military is holding 2,400 people at Abu Ghraib, the prison west of Baghdad where the abuses occurred, and 2,500 at Camp Bucca near the southern port city of Umm Qasr. Earlier this year, Abu Ghraib had close to 7,000 inmates.

Miller said the number of "high-value" reports -- information defined as "actionable intelligence" that could be used almost immediately in operations aimed at stopping insurgents -- had increased from about 200 in July to 325 in August.

Miller's characterization of intelligence gleaned from interrogations contradicted recent warnings voiced by senior military officials in Washington. The officials, including Maj. Gen. George R. Fay, who headed an Army probe of military intelligence soldiers involved in the abuse at Abu Ghraib, have said that the abuse scandal has had a chilling effect on military intelligence-gathering operations because interrogators are being more cautious and detainees have been tipped off to U.S. interrogation methods.

The new procedures are meant to yield useful information and end abuse. Among other new restrictions, interrogators are not allowed to keep prisoners in stress positions, according to U.S. officials. The new procedures also are aimed at improving cooperation between military intelligence and military police, as well as follow-up reviews of interrogations.

A nine-member board composed of six representatives from the Iraqi government and three officers from foreign military forces meets to decide whether there is sufficient evidence to hold detainees, Miller said. Previously, a three-member board consisting only of representatives of the foreign forces reviewed the cases.

The new board has reviewed the cases of 650 detainees since it began meeting Aug. 21, and of those, it has recommended 420 detainees for release, according to the U.S. military. "We think this is a significant step forward, as the Iraqis are making decisions about the returning of Iraqi citizens into society," Miller said.

In Baghdad on Monday, several hundred protesters marched peacefully through streets near the fortified area where the interim government and U.S. Embassy are headquartered. The crowd, consisting mostly of men, displayed banners, some of which read, "Yes for the law. No for the gun militia."

Iraqi police closed off portions of a commercial street to allow the protesters to pass without traffic. "We need to have a conference. We don't need to have weapons," one white banner read. The protesters were mostly former Iraqi army soldiers who say they want to return to their jobs.

A Humvee near Fallujah, in Anbar province, was destroyed in a car bombing. The bomb detonated as a U.S. convoy traveled down a barren stretch of road.U.S. troops collect the bodies of their fallen comrades after the blast. The apparent suicide attack claimed more U.S. troops than any strike since April 29.