Monday's killings happened in the western city of Falluja when the unarmed firefighters got out of their vehicle and were fired upon by US soldiers.
A statement describing the incident said: "After receiving word that a fire truck and its crew were just hijacked, coalition forces pulled over a fire truck matching that description.
"The suspected insurgents were in fact firefighters responding to a call. The fire truck number did not match the one of the hijacked truck. All four of the firefighters that exited the vehicle died.
"Minutes later, another fire truck was spotted and pulled over in an area nearby. The occupants fled. It was determined that this truck number did match the truck that was hijacked."
American soldiers are often accused of using excessive and indiscriminate force in Iraq, although commanders say the problem is now taken more seriously and the number of wrongful killings has diminished.
On Tuesday, two Iraqi policemen were killed by armed men in Amara.
Security sources said Captain Hussein Salah was shot dead at his home in an attack that also left two of his brothers wounded.
Another policeman, Ala Ghleim, was killed under similar circumstances.
The Iraqi army declared a curfew in Shia-dominated Amara on Monday after two days of violence last week in which at least 24 people were killed and the Jaish al-Mahdi (al-Maydi Army) militia burned down two police stations.
Also on Tuesday, the US military announced that three more of its soldiers had been killed in iraq.
Two marines died in the western province of al-Anbar "due to enemy action", the military said.