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Seven inmates were hospitalized on Wednesday night after experiencing what officials described as “a medical emergency” while incarcerated at a jail in northwestern Washington. Authorities believe the inmates may have suffered from fentanyl overdoses, CBS affiliate KIRO-TV reported.

The Everett Fire Department acknowledged that there had been a “multiple casualty incident” involving seven inmates at the Snohomish County Jail, about 30 miles north of Seattle, in a series of tweets as well as a Facebook post. 

“Shortly after 7:30 p.m. this evening, Everett Fire was dispatched to the Snohomish County Jail for a medical emergency involving multiple inmates,” the fire department said. 

Fire officials transported a total of seven patients to a hospital. No deaths were reported in connection with the incident at the time of the Facebook update, according to the fire department. KIRO reported that deputies had observed unusual behaviors in the inmates, which appeared to be consistent with symptoms of an overdose caused by the drug fentanyl.

The Snohomish County Sheriff’s Office is leading an investigation into the incident. CBS News contacted the Everett Fire Department for more information but did not receive an immediate response.

Fentanyl is a synthetic opioid, similar but much more potent than morphine, that is sometimes abused or found laced in other drugs. Its effects can be deadly, and health officials have warned that its role in U.S. drug overdoses is becoming more prevalent.