A suicide bomber rammed his explosives-laden truck into a wall around a police headquarters in northern Iraq on Friday, killing five American soldiers and two Iraqi policemen in the single deadliest attack against US forces this year, the US military and Iraqi police said. A sixth American soldier and 17 Iraqi policemen were also wounded in the blast that took place near the national police headquarters in southwestern Mosul - Iraq's third-largest city and al-Qaida's last urban stronghold. Suicide bombings - a hallmark of al-Qaida's attack style - continue to threaten the city, which US troops must leave by June 30 under an agreement with the Iraqis. The approaching deadline has raised fears about what will happen after American soldiers depart. Lt. Col. Michael Stuart, chief of US operations in Mosul, said the target was the Iraqi national police complex and not the US patrol. He said the American patrol just happened to be on the same street when the attack occurred.