Eighteen guards and other staffers at a central Illinois prison were rushed to hospitals Wednesday after they became sickened while responding to one or more inmates suffering "severe discomfort," according to a prison spokesperson and a prison employees' union representative.

Employees at John A. Graham Correctional Center in Hillsboro became ill while responding to a "medical incident involving individuals in custody who appeared to be under the influence of an unknown substance," state Department of Corrections spokesperson Naomi Puzzello said in a statement to The Associated Press. 

The prison officers and other staff who responded to the emergency call became ill, some violently, when they came near the affected inmates, Anders Lindall of the American Federation of State, County and Municipal Employees Council 31, an employee union, told the AP. 

"Those who were in proximity to the inmate were immediately overcome with a variety of symptoms: Lightheadedness, dizziness, vomiting or feeling nauseous," Lindall said.

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Officials said 18 prison employees required treatment at area hospitals. An undisclosed number of inmates received treatment in the health care unit of the prison about 65 miles northeast of St. Louis.

John A. Graham Correctional Center sign outside

This undated Google street view images shows John A. Graham Correctional Center in Hillsboro, Illinois. (Google Maps)

FOX 2 St. Louis described the situation as a potential "mass overdose" between inmates, citing one law enforcement official who told the station a call went out between local agencies for NARCAN to be rushed to the prison. It wasn't immediately clear which drug was suspected to have caused the sudden illness afflicting the guards upon exposure. 

An Illinois State Police hazardous materials team is investigating, Puzzello said. 

"All staff members who may have potentially encountered the unknown substance were also sent to a local hospital, as a precaution. All staff are stable currently and many have already been discharged," Puzzello said.

Many staff members received medical care at Hillsboro Area Hospital. 

prison bars

This file photo shows an offender holding steel cage jail cell bars.  (iStock)

When the facility reached capacity, others sought treatment at HSHS St. Francis Hospital in Litchfield, 10 miles west. Most were released, but several were kept for observation and testing to determine the cause, Lindall said.

"Hillsboro Area Hospital has provided care to patients that have been transported from Graham Correctional Center," a hospital spokesperson told FOX 2. "Hillsboro Area Hospital received eight patients. All patients are currently in stable condition and will continue to be monitored as needed. The initial 911 page was sent out around 2:38 p.m., and this is an ongoing situation. Hillsboro Area Hospital implemented our mass influx plan. All transported individuals were provided immediate care, including decontamination. The Hillsboro Hospital Administrative Team would like to thank our team members, Local EMS, Law Enforcement Officers, and County EMA for quick assistance."

John A. Graham Correctional Center in Hillsboro

This image shows an aerial view of John A. Graham Correctional Center in Hillsboro, Illinois.  (Google Maps)

AFSCME's state council has expressed growing concern about prison employees coming in contact with dangerous substances.

"The issue of exposure to harmful substances in prisons is increasing," Lindall said, adding that prisons need better searches. "AFSCME has been sounding the alarm for months to tighten up the protocol for both incoming mail and visitor screening."

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Lindall said a union meeting was underway at the prison when the call went out and members broke up the gathering to transport colleagues to the hospital, alert family members and provide other assistance.

Graham Correctional Center is a medium-security lockup for adult males, which opened in 1980 with room for 1,596 inmates. The prison currently houses 1,328 inmates.

The Associated Press contributed to this report.