Editor's note: This story has been updated throughout.
A correctional officer at the French Robertson Unit 10 miles northeast of Abilene was found unresponsive near the kitchen area of the prison early Saturday and later pronounced dead, according to a statement from the Texas Department of Criminal Justice.
"Life saving measures were immediately initiated and 55 year old Correctional Officer Mari Johnson was transported to Hendricks Medical Center in Abilene where she was pronounced deceased," spokesman Jason Clark said in the statement. "The Office of Inspector General – TDCJ is investigating the incident as a homicide."
Investigators with the inspector general's office for the agency were processing the scene on Saturday, Clark said.
"This is a tragic reminder that carrying out that mission can lead to the ultimate sacrifice," said TDCJ Executive Director Brad Livingston in a statement. "Ms. Johnson made the ultimate sacrifice to keep this state safe. She will never be forgotten. We will see that the offender who committed this cowardly act is held accountable."
Gov. Greg Abbott on Saturday afternoon issued a statement in which he said the state of Texas was mourning for Johnson, saying that she "was senselessly murdered."
Abbott added, "Texas will ensure the perpetrator receives swift justice, and TDCJ has been instructed to take all necessary measures to enhance the safety of their staff to prevent such tragedies."
Robertson Unit is a maximum security, all-male facility.
Almost exactly a year ago, rookie prison guard Timothy Davison, 47, was beaten to death by an inmate inside the Barry B. Telford Unit in New Boston in northeastern Texas.
Davison was escorting Billy Joel Tracy, 37, on the morning of July 15 when the handcuffed inmate freed one hand from his restraints and began beating the officer. After pulling him to the ground, Tracy began beating Davison with a two-foot "pry bar" tool guards carry to unlock the small meal tray openings in cell doors.
In 2007, correctional officer Susan Canfield, 59, was killed during an escape attempt at the Wynne Unit in Huntsville.