The investigation began on Monday after 28- year-old inmate Steven Gonzales approached uniformed security officers at the unit to say he was having issues with his cell mate, officials said in a news release.
Gonzales further indicated that officers should go to the assigned cell.
A responding officer approached the cell and found the cell mate lying on the bottom bunk covered with a mattress. He was unresponsive, the release said.
Security and medical personnel initiated life-saving measures, and the victim was rushed to the infirmary on a gurney.
Emergency medical personnel arrived at the scene and pronounced the offender dead at 5:58 p.m.
TDCJ spokesman Jason Clark said Gonzales has been placed in administrative segregation.
The name of the victim was being withheld Tuesday until his family could be notified of the incident.
Gonzales was received by the TDCJ on Jan. 15, 2008, from Dallas County to serve a life sentence for murder.