A man known to police for several run-ins with the law, most recently for waving a Confederate flag at a football game, was arrested as a suspect in the ambush killings of two Iowa officers shot in their patrol cars early on Wednesday, authorities said.

Police offered no possible motive for what precipitated the attacks, which unfolded shortly after 1 am (0600 GMT) in Iowa's capital, Des Moines, and its affluent Urbandale suburb, nor did they explain what led them to the suspect, Scott Michael Greene, 46.

Details of the shootings and the circumstances of Greene's arrest were not made public.

But the body of the first slain officer was found near the stadium where Greene had been expelled by police last month. Greene, who is white, was removed after waving a Confederate battle flag in front of black spectators while the national anthem was being played at the start of a high school football game.

The two slain policemen were both white, each shot while sitting in his cruiser. Urbandale officer Justin Martin, 24, was found dead first, and the body of Sergeant Anthony Bemino, 38, of the Des Moines department, was discovered 20 minutes later.

Three bullet holes were visible in Bemino's patrol car in Des Moines, about 2 miles (3 km) away from the Urbandale shooting scene.

"These officers were ambushed," Des Moines police spokesman Paul Parizek told a news conference.

Urbandale Police Chief Ross McCarty said Greene, who has not been formally charged in the crimes, was well known to local police after various previous encounters with law enforcement besides the flag-waving incident.

In 2014, he pleaded guilty to interference with official acts in an incident involving police.