An al-Qaida affiliate in Iraq claimed responsibility in a statement posted Saturday on a militant Web site for this week's downing of a US helicopter south of Baghdad. The Islamic State of Iraq - which groups al-Qaida in Iraq and several other Sunni extremist groups - said in the statement on an Islamic Web site commonly used by the militants that it was behind the shooting of a US Black Hawk on Thursday south of the Iraqi capital. The statement, whose authenticity could not be independently verified, did not provide details of how the attack was carried out but claimed that "all on board were killed." US Army said in a brief statement Thursday that a helicopter carrying nine personnel went down south of Baghdad but that all on board survived. Four were wounded. The US military did not give details but said the incident was under investigation.