Penal Code section 4573.6 makes it a felony for any person to possess a controlled substance in or within the grounds of specified penal institutions. Ortiz was an employee of Atascadero State Hospital (ASH), charged with this offense for possessing methamphetamine while on ASH grounds. The trial court granted Ortiz’s 995 motion, as ASH was not a penal institution specified by the statute. The prosecutor petitioned for a writ of mandate, which was granted in this opinion. The trial court erred in concluding that mentally ill patients housed at ASH are not held under the custody of peace officers as contemplated by section 4573.6. ASH is a custodial institution with regard to the inmates who are transferred there pursuant to Penal Code section 2684, and their movement within the facility is controlled by peace officers. Therefore ASH qualifies as an institution where inmates are held under the custody of peace officers as contemplated by section 4573.6.