WASHINGTON (AFP) - The US state of Ohio used the last dose of its execution drug on Wednesday to put to death a man who killed his neighbor's boyfriend and a police officer in a 1994 rampage.

After spending two decades on death row, Harry Mitts was declared dead by lethal injection at 10.39am (10.39pm Singapore time) in Lucasville, Ohio, a prisons spokesman said.

The midwestern state used its last dose of the powerful sedative pentobarbital for the 61-year-old's execution. Ohio may announce a new execution method next month.

During his August 1994 shooting spree, Mitts shouted racial slurs and killed a neighbour's black boyfriend, John Bryant, before shooting to death white police officer Dennis Glivar, who was responding to the incident.

Mitts also shot and wounded two other responding police officers.

He had exhausted all his possible appeals up to the US Supreme Court, which confirmed his execution in 2011.

During his trial, Mitts claimed that he was drunk and wanted to die at the hands of police, desperate for a divorce after his wife had left him for a policeman.

The Ohio Parole Board and Governor John Kasich, who has expressed reservations about capital punishment, denied Mitts clemency last month.

"It is apparent that Mitts targeted his first victim, John Bryant, because Bryant was African American," the parole board said.

"Mitts exhibited a complete disregard for the lives of officers and innocent bystanders at the scene." The execution was the 26th this year in the United States and the third in Ohio.