BAGHDAD, Iraq – A suicide attacker sped up to an American military convoy outside Fallujah and detonated an explosives-packed vehicle yesterday, killing seven Marines and three Iraqi soldiers, American military officials said. It was the deadliest day for American forces in four months.

The force of the blast on a dusty stretch of wasteland nine miles north of Fallujah, a hotbed of Sunni insurgents, wrecked two Humvee vehicles and hurled the suicide car’s engine far from the site, witnesses and military officials said.

The bombing underscored the challenges American commanders face in securing Fallujah and surrounding Anbar province, the heartland of a Sunni Muslim insurgency bent on driving coalition forces from the country.

American forces have not patrolled in Fallujah since ending a three-week siege of the city in April that had been aimed at rooting out rebel fighters. Insurgents have only strengthened their hold on Fallujah since then.

Meanwhile, the Interior Ministry said medical tests confirmed that Iraqi authorities had once again mistakenly reported the capture of ousted dictator Saddam Hussein’s deputy, Izzat Ibrahim al-Douri, putting a stop to two days of conflicting statements about his purported arrest.

Ministry spokesman Sabah Kadhim said unspecified tests had shown that a man being held in Iraqi custody was actually a relative of Mr. al-Douri who played only a minor role in Saddam’s regime but was nevertheless wanted by authorities.

The reports on Mr. al-Douri – the most wanted Saddam-era henchman still at large – came as an embarrassment to interim Prime Minister Allawi’s government and exposed a lack of coordination among ministers competing for influence ahead of January elections.

Three soldiers were wounded yesterday in a roadside bombing in eastern Baghdad. After the suicide blast outside Fallujah, west of the capital, medical teams in helicopters ferried away the injured from the blazing wreckage and troops sealed off the area.

Fallujah hospital officials said four Iraqis were wounded by gunfire from American troops near the site of the bombing, but the American military had no confirmation.

The military condemned the bombing as “a desperate act of inhumanity” but insisted American troops will stay the course in Iraq until local forces are in a position to take over security operations. The slain Americans belonged to the 1st Marine Expeditionary Force.

Hours after the attack, an unmanned American spy plane crashed in Fallujah. Afterward, jubilant residents picked up pieces of debris and danced in the streets, displaying pieces of the aircraft to reporters, witnesses said.

Since the Marine siege ended, gunmen have been using the city as a base to manufacture car bombs and launch attacks on American and Iraqi government forces. Fallujah has become a virtual no-go zone for American troops, though American warplanes have repeatedly carried out air strikes against alleged safe houses for rebel fighters there.

Yesterday’s attack resulted in the largest number of Americans killed in combat in a single day since May 2,when nine American troops died in separate mortar attacks and roadside bombings in Baghdad, Ramadi, and Kirkuk.

On Sunday, both the Iraqi minister of state, Qassim Dawoud, and a Defense Ministry spokesman publicly proclaimed Mr. al-Douri’s capture. Later, the defense minister, Hazem Shaalan, said word of his arrest was “baseless.” Mr. Kadhim, the Interior Ministry spokesman, did not, however, identify the al-Douri relative in custody, nor did he say when or where he was captured.

“This will make the government lose credibility after its ministers and top officials appeared to be either liars or foolish,” said Abdel Amir, an Iraqi political analyst and former editor of “Baghdad,” the mouthpiece of Mr. Allawi’s Iraqi National Accord party.

Mr. Amir said the conflicting claims reflected the rivalry within the government between ministers from different ethnic, religious, and political groups, each keen to showcase his own achievements or embarrass rivals.

“It proved that they are very inexperienced and lack coordination on such important issues,” added Mohammed Abdel Jabar al-Shabout, editor of Baghdad’s Al-Sabah newspaper.

Also yesterday, a Turkish driver taken hostage in Iraq was released by his captors, Turkey’s Foreign Ministry said. The release came a day after the driver’s company announced it would withdraw from Iraq in line with his captors’ demands.

Separately, dozens of Iraqi troops surrounded the office of radical Shiite cleric Muqtada al-Sadr in the holy city of Najaf yesterday, but backed off amid pressure from Iraq’s top Shiite cleric, Grand Ayatollah Ali al-Sistani, Mr. al-Sadr’s supporters and witnesses said.