An autopsy by the Office of the Chief Medical Examiner concluded that an inmate killed Tuesday at the Maine State Prison in Warren died of blunt force trauma to the head.

The death of Alan Powell Jr., 57, has been ruled a homicide, a state police spokesman said Thursday. Police had already said they believe Powell was killed by fellow inmate Guy Hunnewell, 42, as the two were in an outdoor exercise yard at the state’s high-security prison in Warren.

Prosecutors said Wednesday that they plan to seek charges against Hunnewell when a Knox County grand jury meets in July.

Police said they would not release any other details.

Hunnewell is serving 40 years for killing his girlfriend. Powell was serving a life sentence for raping, stabbing and strangling a Waterville woman in 1989.

Deputy Attorney General William Stokes said there is no need to arrest and charge Hunnewell immediately because there is no concern that he will flee or commit new crimes.

Powell had served about 22 years of a life sentence for murder.