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A man suspected of shooting dead two police officers as they sat in their patrol cars has had a 'Vote Trump' sign up outside his house for two weeks, neighbours said.

Scott Michael Greene, 46, has a history of run-ins with the law, most recently for flying a Confederate flag at a football game.

Police say they have not established a motive behind the horror attacks which unfolded at around 1am local time in Iowa's capital, Des Moines.

The body of officer Justin Martin, 24, was found near the stadium where Greene, who is white, was kicked out by police last month after waving a Confederate battle flag in front of black spectators while the national anthem was being played.

(Image: AFP PHOTO / Des Moines Police Department / Handout)

The body of Sergeant Anthony Bemino, 38, of the Des Moines department, was discovered 20 minutes later.

Both policemen were white, each shot while sitting in his cruiser.

Three bullet holes were visible in Bemino's patrol car.

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

Greene handed himself into police a few hours later.

Police said he flagged down a state Department of Natural Resources employee on a rural road, presented his ID and asked the employee to call 911.

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Greene was then taken into custody without incident.

He is now being held under guard at a hospital where he was taken after telling officers who arrested him that he was suffering from "some kind of flare-up related to an existing medical condition," Parizek told reporters.

Neighbours told the Des Moines Register that Green had been living with his mother, who moved out of the house after a recent fight with her son that led to her being charged with misdemeanor domestic abuse.

Greene, a military veteran, has a lengthy criminal record and in a 2007 bankruptcy filing said he was single with three children.

In 2001, he was convicted of domestic assault, causing bodily injury and fourth-degree criminal mischief in 2001 among other crimes.

(Image: ABC)

He was then held in 2010 after allegedly driving under the influence and in 2014 on a charge of first-degree harassment.

Speaking at a press conference, Sergeant Paul Parizek branded the attacker a "coward" and revealed that officers have been ordered to patrol in pairs after the city's first police shooting since 1977.

He said: "In all appearances it looks just like that, that these officers were ambushed.

(Image: https://www.washingtonpost.com/news/morning-mix/wp/2016/11/02/two-iowa-police-officers-killed-in-what-police-are-calling-ambush-style-attacks/) (Image: Google)

"It doesn't look like there was any interaction between these cowards and the officers they shot in their cars.

“These guys were gunned down, sitting in their car doing nothing wrong.

"There's somebody out there shooting police... there is a clear and present danger to police officers right now."

“We’re very well aware of the society that we’re living in right now and the time and that there are some not so positive views of law enforcement that a segment of our population holds.

“If we don’t provide the service in the area that we do, with the personal-type service that we do, we’re nothing more than an occupying army.

"We’re going to do what we can to keep ourselves safe.”

Earlier this year three police officers were killed in Baton Rouge in Louisiana after a man targeted them with "chilling, sheer brutality".

The officers were responding to a call of shots fired when they were ambushed by a gunman.

Attacker Gavin Eugene Long, 29m had called for black people to "fight back" against police "devils" just days before the horror shooting incident.

Footage emerged of the former sergeant ranting about the treatment of African-American protesters by police – and urging them to "shed blood".