BAGHDAD, Iraq (AP) – In one of their boldest and most brutal attacks 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 down on the ground and then 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 over precise figures, although the Iraqi National Guard said 48 troops and three drivers were killed.

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.

U.S. officials believe al-Zarqawi’s group is headquartered in Fallujah, an insurgent bastion 40 miles west of Baghdad. On Sunday, a U.S. Marine F-18 Hornet jet struck an insurgent position there, the U.S. 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. U.S. 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 Sunday but the cause could not be determined.

Iraqi police and soldiers have been increasingly targeted by insurgents, mostly 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 guerrillas may have had advance word on the soldiers’ travel.

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

Last week, a U.S. defense official told reporters in Washington that some members of the Iraqi security services have developed sympathies and contacts with the guerrillas. 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 U.S. forces could be drawn down.

One American soldier also was wounded in the pre-dawn attack that killed Seitz, the State Department official. The attack occurred at Camp Victory, the headquarters of the U.S.-led coalition’s ground forces command.

Seitz was believed to be the first full-time State Department officer killed in Iraq. Last October, a female U.S. Foreign Service officer was severely wounded in the arm in a rocket barrage on the Rasheed Hotel.

Deputy Defense Secretary Paul Wolfowitz, one of the architects of the Iraq war, was in the hotel at the time but escaped injury.

In Beijing, Secretary of State Colin Powell described Seitz as “a brave American, dedicated to his country and to a brighter future for the people of Iraq.”

U.S. Ambassador John Negroponte said Seitz was a “committed professional” who served with distinction.

“He came to Iraq, as did his fellow Americans here, to help the Iraqis defeat terrorism and the insurgency, establish democracy, and rebuild their economy,” Negroponte said.

Elsewhere, insurgents attacked Iraq security patrols three times late Saturday in Samarra, 60 miles north of Baghdad, wounding two Iraqi soldiers, the U.S. military said.

Last month, U.S. and Iraqi forces recaptured Samarra from insurgents but have faced car bombings and scattered attacks ever since.

Militants also targeted Iraq National Guard forces near Baqouba, wounding seven in bomb attacks that began Saturday, officials said. Three guardsmen and their driver were wounded Sunday while three others were injured Saturday.

In Karbala, Muslim al-Taie, an aide to senior Shiite cleric Hussein al-Sadr, was killed in a drive-by shooting. One of al-Taie’s bodyguards also was killed and another was injured, a Karbala city council official said.

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Associated Press reporters Rawya Rageh in Baghdad, Hanna Daghestani in Baqouba and Abdul Razzak Jabr in Kut contributed to this report.

AP-ES-10-24-04 1807EDT