A spokesman for the Sinaloa state prosecutors' office says all of the victims were shot to death. Investigators have found two pistols and an assault rifle in the prison in the Pacific coast city of Mazatlan.
Two police officers guarding the prison were wounded in the Monday shootings, one seriously.
Elsewhere, in an attack near a high school in western Mexico.
The Public Safety Department says police officers came under fire from a gang near a vocational high school in Michoacan state.
Several police were wounded in the attack Monday.