Al-Qaida-linked Sunni insurgents claimed responsibility Wednesday for the downing of a US military helicopter northwest of Baghdad. The claim was issued in an Internet statement signed by the Islamic State in Iraq, an umbrella group of several Sunni insurgent groups, including al-Qaida in Iraq. The authenticity of the statement, posted on a Web forum where the group often issues statements, could not be independently confirmed. The US military said a CH-46 Sea Knight helicopter went down about 30 kilometers (20 miles) northwest of the capital. An Iraqi air force officer said the helicopter was downed by an anti-aircraft missile. The US military did not give the cause of the crash or comment on casualties, though one US military official said there were reportedly fewer than 10 people on board. "By God's grace, the downing of a helicopter of the crusaders was accomplished on Wednesday morning near the Karamah in Anbar province, leading to its destruction and the deaths of all those on board," the Islamic State in Iraq statement said. It identified the downed helicopter as a Chinook. The Marine Corps' Sea Knight transport helicopter, with its pair of rotors, resembles the Army Chinook, though is slightly smaller. The statement said the group would issue a video of the copter's downing in a later message.