Associated Press WriterJEFFERSON CITY, Mo. (AP) -- Law officers were searching for two missing convicted murderers Thursday after a third inmate was found dead in the prison ice plant where all three had been working with sharp tools and without any direct supervision.

The dead inmate was identified by prison officials as Toby Viles, 28, who was serving three concurrent life sentences for the 1992 slayings of his siblings.

Viles was found dead with wounds to his head and neck about 10 p.m. Wednesday, shortly after the scheduled end of his icehouse work shift with fellow convicted murderers Chris Simms and Shannon Phillips, both of whom were missing Thursday, said Department of Corrections spokesman Tim Kniest.

"We certainly are investigating the death as a homicide," he said. "We don't have a motivation at this time."

The three inmates were the only ones assigned to the ice plant and were not under the direct supervision of a guard, although they were locked in the building and occasionally checked upon, Kniest said. As part of their work, the inmates routinely used chisels and hammers to chop up ice, Kniest said.

Investigators were trying to determine the cause of Viles' death, as well as whether any of the icehouse tools were missing, he said.

About 200 prison officials, including emergency specialists from five other area prisons, were searching for the inmates inside the expansive 47-acre prison site, which has about 25 buildings plus numerous underground steam tunnels, Kniest said.

Local and state law enforcement officers were searching outside the facility, "but we are still focusing our search with the Department of Corrections inside the facility, because we have not received any confirmed sightings outside the facility and we have not found any evidence that would lead us to believe the inmates left the facility," Kniest said Thursday morning.

Viles, who had been imprisoned since Sept. 14, 1994, was 16 years old when he killed his siblings -- Tabitha Viles, 14, Quincy Viles, 11, and Tristan Viles, 3 -- at their Laclede County home near Lebanon.

All three children were shot while eating ice cream on March 17, 1992. The parents were not home at the time, and after the shootings Viles went to a nearby school and got a friend to come back to the house with him. The two then told a neighbor, and police were called.

Viles was certified to be tried as an adult, and because of extensive publicity about the case it was transferred to Warrensburg in Johnson County. He subsequently pleaded guilty to the three first-degree murder charges, and the state agreed not to seek the death penalty and dropped additional charges of armed criminal action.

Simms, 27, has been in prison since September 1998, serving a life sentence and a concurrent 10 years for first-degree murder and armed criminal action in St. Louis.

Phillips, 35, was serving a life sentence for a first-degree murder conviction in Jackson County, and a consecutive life sentence for armed criminal action. He had been imprisoned since April 1996.

The three inmates were scheduled to work from 3 p.m. to 9 p.m. at the icehouse, Kniest said. Simms reported for dinner at 5:30 p.m. but the other two did not, which was not necessarily unusual because inmates sometimes skip meals, he said. A prison staff member later talked with one of the three over the phone, but Kniest did not know whom or the precise time. Viles was found dead after the three failed to respond to a routine call to return to their housing units.

Kniest said there was no prior indication of any problems among the three, who had worked in the ice plant for about a year.

The Cole County sheriff's office, Jefferson City police and the Missouri State Highway Patrol were also assisting in the investigation and search.

The last time an inmate was killed at the Missouri State Penitentiary was February 1990, and it is also been more than a decade since someone escaped the prison, Kniest said.