Baghdad ? Two dump trucks sped down the road, then took aim at an outpost defended by paratroopers of the 82nd Airborne Division.

A soldier carries two rocket launchers seized Tuesday in a raid in the Ameriyah neighborhood of Baghdad. Two men were detained after soldiers found a sizeable weapons cache in their home.

Under heavy gunfire, one of the heavy vehicles smashed into an outer ring of concrete barriers and exploded. The second rammed into the wrecked truck, dragging it and other rubble before it exploded 30 yards from the building housing the post’s troops, collapsing two walls.

Nine American soldiers lay dead and 20 others were injured, U.S. officials said Tuesday as an al-Qaida-linked group claimed it used “new methods” in staging the attack in volatile Diyala province.

The assault underscored the ability of guerrillas of the Sunni Arab-dominated insurgency to wage war in Iraq four years after the U.S.-led invasion, and it came in a region that has seen violence escalate since U.S. and Iraqi troops launched the security crackdown in Baghdad.

All the casualties were in the 5th Squadron, 73rd Cavalry Regiment, 3rd Brigade Combat Team, which has been conducting operations in largely impoverished villages as part of a security push to tame insurgent activity in Diyala.

The deaths made April the deadliest month of the year for the U.S. military. It was also the single deadliest attack on U.S. ground forces since Dec. 1, 2005, when 10 Marines were killed by a bomb inside an abandoned flour mill near Fallujah.

“We are recovering, supporting the families during this time of loss, praying for them and continuing our mission,” Lt. Col. Michael Donnelly, U.S. military spokesman in north Iraq, told The Associated Press in telephone interview. “The enemy brings nothing to benefit the people – nothing.”

Monday’s attack at the small town of Sadah inflicted the biggest loss on the 82nd Airborne since June 1969, when 12 paratroopers were ambushed and killed in Vietnam, a spokesman, Maj. Tom Earnhardt, said at the division’s base at Fort Bragg, N.C.

Donnelly said the patrol base was set up less than a month ago after an operation that sought to drive militants from the area. Sadah, a rural Sunni town of about 7,000 people near the capital of Diyala province, Baqouba, has been an al-Qaida stronghold.

The Islamic State of Iraq, an umbrella group of Sunni militants that includes al-Qaida in Iraq, said it was behind the suicide attack. Its account on the Internet was similar to that of the U.S. military, but claimed it used new techniques.

“Almighty God has guided the soldiers of the Islamic State of Iraq to new methods of explosions,” the statement said without elaborating, while claiming 30 Americans died.

Lt. Col. Christopher Garver, a U.S. military spokesman in Baghdad, said the style of the attack fit the pattern of al-Qaida but he said an investigation was under way into who was to blame and exactly what happened.

When asked about the “new methods” claimed by the group, he said the military was on heightened alert for dump trucks as they had been used in several recent high-profile attacks.

“The use of dump trucks seems to be a recurring theme recently in the last few weeks,” he said.

He said the military had foiled several attempts to use the large vehicles, mentioning the discovery earlier this month of a dump truck filled with barrels of gasoline that overturned north of Baghdad.

Michael O’Hanlon, a military analyst at the Brookings Institution, said he didn’t think the attack demonstrated a particular new technology, adding that securing remote bases in Iraq has always been difficult for U.S. troops.

“Small arms fire followed by two truck bombs is not new, it’s just effective use of time-tested tactics,” he said in an e-mail.