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A man died Saturday and 12 people were hospitalized in what authorities in Northern California described as a mass casualty overdose on the powerful narcotic fentanyl.

Four of the victims were in critical condition, said Mike O'Brien, a police captain in Chico, California.

"Certainly there’s potential for additional fatalities," he said at a news conference. "I want to emphasize that."

Two police officers who responded became ill and were treated and released from a hospital, O'Brien said.

Chico Fire Department Chief Steven Standridge said the officers were "potentially exposed" to the drug, a synthetic opioid often imported on the black market from China and Mexico to be used as a filler in heroin and other street narcotics.

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"It was a large, mass casualty incident for us," the fire chief said in a news conference Saturday night.

O'Brien said a person affiliated with the house where the overdoses occurred called 911 about 9 a.m. Saturday to report the incident. Responding officers gave patients six doses of the opioid antidote naloxone while also administering CPR, he said.

Chico officers began carrying the antidote only last year. "It certainly would have been far worse without the response and dispensing of naloxone by Chico police officers," O'Brien said.

It's not clear what drug the fentanyl might have been paired with, he said, but his officers have only come across it when it was combined with heroin.

"Every indication is that that this mass overdose incident was caused from the ingestion of some form of fentanyl in combination with another substance, although that is yet to be confirmed," O'Brien said.

It appeared the victims, believed to be ages 19 to 30, all knew each other, the police chief said.

"The residence where this incident occurred is being treated as a hazmat site," he said.

The conditions of the non-critical victims were not released, and the person who died was not identified. O'Brien said a narcotics task force was trying to determine the source of the drugs.

Fentanyl is "50 to 100 times more potent than morphine," according to the Centers for Disease Control. The drug has been blamed for a wave of overdose deaths across the nation.