A Harris County jail inmate awaiting trial on a murder charge allegedly beat a young illegal immigrant accused of robbing and fatally shooting a 14-year-old girl in the back as she was walking home on Aug. 6.

Melvin Alvarado, 22, one of two men charged with capital murder in the death of Shatavia Anderson, was beaten Thursday evening, sustaining serious enough injuries to be hospitalized and receive stitches, said Alan Bernstein, of the Harris County Sheriff's Office.

Accused in Alvarado's beating is Robert Williams, 38, Bernstein said.

A volunteer inmate crew at the county's jail on San Jacinto entered Alvarado's cell about 5:30 p.m. Thursday to clean it during the one hour a day he is allowed out of it.

Williams "pushed his way" inside the cell and assaulted Alvarado for "a few moments," Bernstein said.

A detention officer pulled Williams off Alvarado, but not before Williams beat him, leaving him with cuts, swelling, bruising and injuries to a right eye and left ear, Bernstein said. He was unsure which particular injury required sutures or how many were required.

Alvarado, who is in the U.S. illegally and has been twice deported, was taken to LBJ Hospital for treatment. Alvarado was back in a cell Saturday — at a different jail, on Baker - and treated in the medical unit there, he said.

The Harris County District Attorney's Office did not bring charges against Williams because he already is facing a higher-level murder charge, Bernstein said.

Williams will, however, face the jail system's in-house discipline, under which he could lose credit for good time served, commissary privileges and visitation, Bernstein said.

Also charged in the girl's death is Jonathan Lopez-Torres, 18, a native of Honduras and a lawful U.S. resident.

Shatavia's family earlier this week publicly called for tighter restrictions on immigration.

The body of the girl, who would have been a freshman at Nimitz High, was discovered last Saturday morning in a field adjacent to the apartments where she lived in the 1100 block of Langwick.

paige.hewitt@chron.com