ATWATER – Federal Bureau of Prisons officials say they are expanding the availability of protective vests after the fatal stabbing of a correctional officer.

The two inmates accused of attacking 22-year-old Jose Rivera June 20 at the U.S. Penitentiary in Atwater have been transferred and are under investigation.

Union officials have argued that a vest might have stopped the knife that penetrated Rivera's heart.

Federal prisons spokesman Mike Truman says the vests are part of several potential changes the bureau is studying to improve safety.