BAGHDAD, Iraq – In their boldest and deadliest ambush yet, insurgents waylaid three minibuses carrying American-trained Iraqi soldiers heading home on leave and massacred about 50 of them – forcing many to lie down on the ground and shooting them in the head, officials said yesterday.

Some accounts by police said the rebels were dressed in Iraqi military uniforms.

The killing of so many Iraqi soldiers – unarmed and in civilian clothes – in such an apparently sure-footed operation reinforced American and Iraqi suspicions that the country’s security services have been infiltrated by insurgents.

A claim of responsibility posted on a terrorist Web site attributed the attack to followers of Jordanian-born terror mastermind Abu Musab al-Zarqawi.

Elsewhere, an American diplomat was killed yesterday when a rebel-fired rocket or mortar shell crashed into the trailer where he was sleeping at an American base near the Baghdad airport, the American Embassy announced.

Edward Seitz, 41, an agent with the State Department’s Bureau of Diplomatic Security, was believed to be the first American diplomat killed in Iraq since the war began in March 2003. Al Jazeera TV reported yesterday that the terrorist Islamic Army of Iraq claimed responsibility for the attack.

A Bulgarian soldier was killed and two others were injured in a car-bombing near Karbala, the Bulgarian Defense Ministry said. Karbala, a Shiite holy city south of Baghdad, has been quiet for months after American troops routed Shiite rebels there last spring.

The Iraqi soldiers were killed on their way home after completing a training course at the Kirkush military camp northeast of Baghdad when their buses were stopped Saturday evening by rebels near the Iranian border about 95 miles east of Baghdad, Interior Ministry spokesman Adnan Abdul-Rahman said.

There was confusion over precise figures, although the Iraqi National Guard said 48 troops and three drivers were killed.

Abdul-Rahman said 37 bodies were found yesterday on the ground with their hands behind their backs, shot execution-style. Twelve others were found in a burned bus, he said. Some officials quoted witnesses as saying insurgents fired rocket-propelled grenades at one bus.

“After inspection, we found out that they were shot after being ordered to lay down on the earth,” General Walid al-Azzawi, commander of the Diyala provincial police, said, adding that the bodies were laid out in four rows, with 12 bodies in each row.

In a Web site posting, Al Qaeda in Iraq, formerly known as Tawhid and Jihad, claimed responsibility for the ambush, saying, “God enabled the Mujahedeen to kill all” the soldiers and “seize two cars and money.”

The claim could not be verified but appeared on a Web site used in the past by Islamic terrorists.

Mr. al-Zarqawi and his movement are believed to be behind dozens of attacks on Iraqi and American-led forces and kidnappings of foreigners. Many of those hostages, including three Americans, have been beheaded – some purportedly by Mr. al-Zarqawi himself.

America has put a $25 million bounty on Mr. al-Zarqawi – the same amount as for Osama bin Laden.

American officials believe Mr. al-Zarqawi’s group is headquartered in Fallujah, a terrorist bastion 40 miles west of Baghdad. Yesterday, an American Marine F-18 Hornet jet struck an insurgent position there, the American military said. Witnesses said six people were killed.

Fallujah fell under rebel control after the Bush administration ordered Marines to lift their three-week siege of the city in April. American commanders have spoken of a new offensive to clear rebel strongholds ahead of Iraq’s crucial elections in January.

Scattered explosions rumbled through central Baghdad late yesterday but the cause could not be determined. Iraqi police and soldiers have been increasingly targeted by insurgents, primarily with car bombs and mortar shells. However, the fact that the insurgents were able to strike at so many unarmed soldiers in such a remote region suggested the rebels may have had advance word on the soldiers’ travel.

“There was probably collusion among the soldiers or other groups,” Diyala’s deputy Governor Aqil Hamid al-Adili told Al Arabiya TV. “Otherwise, the gunmen would not have gotten the information about the soldiers’ departure from their training camp and that they were unarmed.”

Last week, an American defense official told reporters in Washington that some members of the Iraqi security services have developed sympathies and contacts with the rebels. In other instances, infiltrators were sent to join the security services, the official said, on condition of anonymity.

He cited a mortar attack Tuesday on an Iraqi National Guard compound north of Baghdad as a possible inside job. The attackers apparently knew when and where the soldiers were gathering and dropped mortar rounds in the middle of their formation. At least four Iraqis were killed and 80 wounded.

The extent of rebel infiltration is unknown. However, it raises concern about the American strategy of handing over more and more responsibility to Iraqi security forces so American forces could be drawn down.

[The Iraqi government has warned American and international nuclear inspectors that almost 380 tons of powerful conventional explosives are missing from a former military facility, according to the New York Times. Al Qaqaa, the military base, was supposed to be under American control but is currently deserted. White House and Pentagon officials acknowledge that the weapons went missing after the war began last year, the Times reported on its Web site last night.]