Scott Greene Polk County booking photo.

The man accused of killing two central Iowa police officers has pleaded not guilty to two counts of first-degree murder.

Scott Michael Greene of Urbandale signed the written plea that was filed by his attorney Wednesday. The 46-year-old Greene is charged in the November 2 shooting deaths of Urbandale Police Officer Justin Martin and Des Moines Police Sergeant Anthony Beminio.

Police say both officers were killed 20 minutes apart in ambush style attacks as they sat in their patrols cars. Police and Urbandale School officials had a history of confrontations with Greene, who has struggled with mental health issues, according to police and court records.