SAN ANTONIO -- A prison inmate was killed and 22 others were hurt following a riot Friday at a federal prison in Three Rivers.

Fifteen of the injured were hospitalized. Five of them were transported to University Hospital in San Antonio. Seven others were treated at the prison.

No prison staff members were injured.

"The cause of this incident is unknown at this time and an investigation has begun," prison spokesman Dennis Molina said. "There were no staff injuries."

Prison officials said two fights broke out at the prison around 7 a.m. There was no immediate word what sparked the fights, which were brought under control quickly.

How the inmate died is still under investigation, but prison officials don't believe that guards used any potentially lethal force to break up the fight. A prison spokeswoman says 38-year-old Servando Rodriguez was serving a 4 1/2 year sentence for violating terms of his supervised release. The violations included marijuana possession with intent to distribute and being in the U.S. illegally after deportation.

Prison officials said homemade knives were the only weapons used in the incident.

A union spokesman said declined to call the incident a riot, but did call it a gang-related disturbance.

The prison is a medium security facility that houses 1,200 male inmates.

Three Rivers is located about 80 miles south of San Antonio.