LINCOLN, Neb. (AP) — Officials say they tightened security and further restricted prisoner movement after discovery of drug use by some inmates at the Nebraska State Penitentiary in Lincoln.

The Correctional Services Department said in a news release Sunday that at least 15 inmates are suspected of using K2, synthetic marijuana.

The prison canceled all Sunday visits and modified operations to more closely control inmate movement.

Department spokeswoman Dawn-Renee Smith says one inmate required outside medical attention Sunday related to the drug use. Smith says investigators are trying to find out how the K2 got into the prison.