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 Qaeda's last urban stronghold.
Suicide bombings - a hallmark of al Qaeda's attack style - continue to threaten the city, which U.S. 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 U.S. operations in Mosul, said the target was the Iraqi national police complex and not the U.S. patrol. He said the American patrol just happened to be on the same street when the attack occurred.
"It was just bad timing," Stuart told The Associated Press.
Friday's blast was the deadliest single bombing attack against U.S. troops this year. A suicide car bomb struck a U.S. patrol in Mosul on Feb. 9, killing four American soldiers and their Iraqi interpreter. Also, four U.S. soldiers were killed Jan. 26 when two helicopters collided over the northern city of Kirkuk.
The suicide bomber made a sharp turn as he approached the police complex, then rammed his truck through an iron barrier, hitting a sandbagged wall beyond it and detonating his vehicle near the station's main building, Iraqi police said. The blast shook the entire complex, according to witnesses and police.
The U.S. military said two people were detained in connection with Friday's attack, which is under investigation, and the names of those killed were being withheld pending notification of families.
Although U.S. combat troops have to leave Iraqi cities by the end of June under the U.S.-Iraqi security agreement that went into effect this year, the top U.S. commander in Iraq, Gen. Raymond Odierno, told The Times of London this week that the American troops may have to stay in Mosul and another northern city, Baqouba, after the deadline because insurgents remain active there.
The policemen wounded in Friday's blast were taken to a nearby hospital, said a Mosul police officer with the provincial command. He spoke on condition of anonymity because he was not authorized to talk to the media.
Mosul had been relatively quiet in recent weeks compared to Baghdad where attacks killed at least 53 people this week.