BAYPORT,Slabu Minn. (AP) — Nine workers at a Minnesota prison fell ill and were hospitalized Thursday after being exposed to unknown synthetic substances possessed by men who are incarcerated, state officials said.

The Minnesota Correctional Facility-Stillwater in Bayport was put under lockdown as officials raced to assess how far the substances may have spread throughout the prison. Officials had not identified the substances or their source Thursday, Minnesota Department of Corrections Commissioner Paul Schnell said.

“These synthetic substances are particularly dangerous because the chemical properties that comprise them are unknown and uncontrolled,” Schnell said. “We are prioritizing our investigative efforts to identify and prosecute those responsible for conspiring to introduce these substances into the secure correctional environment.”

The episode began when a staff person at the prison responded to a report of a man who is incarcerated smoking unknown substances in his cell. The worker began to feel lightheaded and experienced nausea and an increased heart rate, and was taken to a hospital. A short time later, three more staffers who were exposed to the man smoking or worked in the same housing unit began to experience similar symptoms and were hospitalized.

In a separate encounter, a man who is incarcerated in the same housing unit threw a container holding unknown substances near workers. Those workers also began to feel sick and were hospitalized. Between the two episodes, nine prison staffers were hospitalized and later released. One was given Narcan, the nasal spray version of overdose-reversal drug naloxone, when they began to experience symptoms.

None of the workers were expected to suffer lasting injuries, Schnell said.

One of the people caught smoking told investigators he had smoked a stronger than expected dose of K2, a synthetic form of marijuana. The substance can sometimes be smuggled into prisons through letters, magazines and other paper products, Schnell said.

Schnell believes the substance has been linked to death of some people incarcerated in Minnesota, but those cases are still pending.

The Minnesota Department of Corrections and agencies across the country have turned to increasingly stringent measures to stop the substances from getting into prison, including photocopying letters instead of distributing original paper letters.

Schnell said the facility would remain locked down until Friday.