An al-Qaida-linked group claimed responsibility Tuesday for a suicide truck bombing that killed nine US paratroopers in the worst attack on American ground forces in Iraq in more than a year, saying it sent "two knights" for the attack. The Islamic State of Iraq, an umbrella group of Sunni insurgents that includes al-Qaida in Iraq, said it was behind Monday's double attack on a US patrol base in Diyala province northeast of Baghdad - an area that has seen violence spike since American troops surged into the capital to halt violence there. The US military only confirmed one truck bombing. "The first knight exploded his truck on them and he was followed by his brother in the second truck, exploding it on what remains from the soldiers inside the headquarters," said the statement, posted on an insurgent Web site. The victims were all members of the Army's 82nd Airborne Division, said a spokesman for the Fort Bragg, N.C.-based unit. It was the highest number of casualties for the division since the war began, Maj. Tom Earnhardt said. "We are recovering, supporting the families during this time of loss, praying for them and continuing our mission," said Lt. Col. Michael Donnelly, the US military spokesman in northern Iraq. "The enemy brings nothing to benefit the people - nothing." In its Web posting Tuesday, the Islamic State of Iraq, an insurgent umbrella group that includes al-Qaida, put the number of Americans killed at 30. "Almighty God has guided the soldiers of the Islamic State of Iraq to new methods of explosions," it said without elaborating. The message appeared on a Web site that frequently airs communications from operatives, but its authenticity could not be independently confirmed. It was single deadliest attack on ground forces since December 1, 2005, when a roadside bomb killed 10 Marines and wounded 11 on a foot patrol near Fallujah. Twelve soldiers died when a Black Hawk helicopter crashed in Diyala on January 20. The military said it might have been shot down but the investigation is still ongoing. The use of a suicide bomber in a direct assault against US forces was unusual. Operatives, seeking to avoid American firepower, have mostly used hit-and-run ambushes, roadside bombs or mortars on US troops.