Two more inmates have died at Crossroads Correctional Center, the state's maximum-security prison in Cameron.
Albert Newton, 29, was pronounced dead at 11:19 a.m. last Monday, said Mike Kemna, prison superintendent.
Mr. Newton entered the Missouri Department of Corrections in February to serve a nine-year sentence on a conviction for second-degree burglary and domestic assault from Howell and Dent counties in southeast Missouri.
The death will be investigated, and an autopsy will be performed to determine the cause of death.
"The department has no cause to believe that foul play is involved," said Lt. Don Fritz, a Cameron Police Department detective. "But we're investigating to make sure."
Robert Endres, 69, died Nov. 29 in the prison's transitional care unit. Mr. Endres died of apparent natural causes following a lengthy illness, Mr. Kemna said.
Mr. Endres had served 20 years on a life sentence without parole for a murder conviction from St. Louis.
Another inmate, Tracey Patterson, 33, died on Nov. 21, and Cameron Police are continuing a criminal investigation. 
"Mr. Patterson's death is an ongoing investigation where foul play is suspected," Mr. Fritz said.
The Cameron police are waiting for autopsy results, Mr. Fritz said.
Six inmates have died at Crossroads this year, said Tim Kniest, a spokesman for the corrections department.