Baghdad, Iraq ? In their boldest and deadliest ambush yet, insurgents waylaid three minibuses carrying U.S.-trained Iraqi soldiers heading home on leave and massacred about 50 of them — forcing many to lie on the ground before shooting them in the head, officials said Sunday.

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 an Islamist Web site attributed the attack to followers of Jordanian-born terror mastermind Abu Musab al-Zarqawi.

Elsewhere, a U.S. diplomat was killed Sunday morning 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 U.S. Embassy announced.

Edward Seitz, 41, an agent with the State Department’s Bureau of Diplomatic Security, was believed to be the first U.S. diplomat killed in Iraq since the war began in March 2003. Al-Jazeera television reported Sunday that the militant 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 U.S. troops routed Shiite militia 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 about precise figures, although the Iraqi National Guard said 48 troops and three drivers were killed.

Iraqi National Guardsmen stand by the bodies of fellow Iraqi soldiers who reportedly were the victims of an insurgent ambush. The bodies of about 50 Iraqi soldiers were found on a remote road in eastern Iraq, where they were attacked as they were heading home on leave, Iraqi authorities said Sunday.

Abdul-Rahman said 37 bodies were found Sunday 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,” Gen. 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, the al-Qaida 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 extremists.

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

The United States has put a $25 million bounty on al-Zarqawi — the same amount as for Osama bin Laden.