A sheriff's spokesman said Jonathan Najera from Ventura was found dead from asphyxiation due to neck compression on March 1.
Investigators are not sure exactly when Najera was killed, because deputies who checked his cell during the night thought he was just sleeping.
His cellmate, whose name has not been released, has admitted to the killing.
Authorities said no weapons were found in the cell.
Najera had recently been sentenced in an attempted murder case.
The Associated Press contributed to this report.