Clashes between gunmen and the police erupted in western Mosul, resulting in the death of a gunman and the wounding of two policemen.
Sunni Arabs make up most of western Mosul, but Kurds dominate the eastern half.
Police stations in western Mosul have perhaps several hundred officers in an area that is supposed to have several thousand.
Traveling back to its base, the patrol was attacked in what American officials described as a milelong ambush along a main thoroughfare in western Mosul.
A suicide car bomb in western Mosul also killed five people, the police said.
Sunni Arabs, who live mostly in western Mosul, are believed to lead the city's insurgency.
The discoveries occurred after nine Iraqi Army soldiers had been found dead in western Mosul on Saturday with bullet wounds to their heads.
A roadside bomb in western Mosul killed at least one, and three bodies were found, the authorities said.
It is calmer now: Attacks against troops in western Mosul fell in July to the lowest level of the year.
The Americans rushed to a mosque where the projectiles were launched, next to the busiest market in western Mosul.