(CNN) Six staff members at a prison in Connecticut were taken to local hospitals for possible exposure to fentanyl, the potent synthetic opioid, the Connecticut Department of Correction said.

An ambulance was called to the Cheshire Correctional Institute, a state prison, at 9 a.m. on Tuesday to take a correctional officer in medical distress to the hospital, the department said in a statement.

About an hour later, a staff member from the facility's school system also began feeling unwell, the statement said, and then a school teacher and three more correctional officers reported feeling poorly.

All six staff members remained conscious and alert, and two have since been treated and released from the hospital, the statement said. No offenders exhibited any signs of fentanyl exposure.

The prison has been placed on lockdown, and the Connecticut State Police Hazmat team and the Department of Energy and Environmental Protection have reported to the facility, the statement said. The Department of Correction and Connecticut State Police are investigating.

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