ANAMOSA, Iowa (WHO) — An inmate and three staff members at the Anamosa State Penitentiary had to be taken to the hospital after being exposed to an unknown substance over the weekend.

The Iowa Department of Corrections said the incident happened Saturday night around 11:30 when security and medical staff responded to an inmate who was found unresponsive in his cell. DOC officials say it appeared the inmate was under the influence of an unknown substance, though no obvious signs of substances were observed.

Medical staff administered Narcan, an opiate reversal drug, and decided the inmate needed to be taken to the hospital for further treatment. The inmate is now back at the prison.

While the inmate was being prepared for transfer to the hospital, three of the staff members who responded became ill. The on-site medical team administered Narcan to two staff members and the DOC said all three were transported to a local hospital for observation, but have been released.

The Department of Public Safety Narcotics Enforcement Division is working with the DOC to investigate the incident.