Two convicted murderers missing from a state prison carefully planned their disappearance — stocking up on food and clearing out their cells before killing a fellow inmate and fleeing, law officers said Friday.

The missing items are evidence that the death of convicted murderer Toby Viles (search) on Wednesday evening was part of an elaborate plan, authorities said as the search for Christopher Sims (search) and Shannon Phillips (search) continued.

"This was not a crime of instant passion," Cole County Sheriff John Hemeyer said Friday. "Whatever happened to this point was something that they were anticipating and planning on, whether it be an escape or some other type of event."

Despite the efforts of 200 searchers now in their third sweep of the 47-acre Missouri State Penitentiary (search), it remained unclear Friday whether Sims and Phillips had escaped or were still hiding on the prison grounds, said Tim Kniest, a Department of Corrections spokesman. There were no confirmed sightings of the men outside the prison.

"We've got even odds — inside, outside — at this point," Hemeyer said.

A note left beside Viles' body and initialed by the missing inmates claimed responsibility for his death and threatened to kill anyone who attempted to confront them, Hemeyer has said. Viles was killed in a prison ice plant, where all three had been working without direct supervision.

Preliminary autopsy results released Friday indicated Viles died of a blow to the head, Kniest said. A pick-ax-type shovel used to chop and move ice was missing from the work site and could have been used, Hemeyer said.

Gone from the missing inmates' cells were their bed sheets, blankets and valuable items, Hemeyer said. The two prisoners also emptied out their inmate bank accounts, purchasing packaged meats, candy bars and other food from the prison store, he said.

The inmates also may have had access to civilian clothes or Corrections Department uniforms, Hemeyer said.

"The guys had plenty of time to plan, and it looks like from what I have been given it was very well planned," said Capt. Ray Bledsoe, a spokesman for the Jefferson City Police Department.

Prison officials last had contact with Viles during a telephone call to the ice plant about 6:10 p.m. Wednesday. His body was not discovered until about 10 p.m.

Kniest said there were no employee shift changes during that four-hour period and it was unlikely that any vehicles entered or left the prison. But he said many prisoners were free to walk around at the time.

Viles was sentenced to life in prison for killing his three younger siblings as a teen. Sims and Phillips were serving life terms for separate killings.

Gov. Bob Holden (search) expressed concern Friday that convicted murderers had been allowed to work alone with potentially lethal tools. Prison officials have called the practice a necessity for a facility reliant on inmate labor.