Police in Northern California responded to a “mass casualty overdose” situation on Saturday that resulted in one death and a dozen people hospitalized.

Authorities said that the incident was caused by exposure to the deadly opioid fentanyl, which also affected two responding officers, according to NBC News.

Mike O’Brien, a police captain in Chico, Calif., said at a news conference that four of the victims were in critical condition.

“Certainly there’s potential for additional fatalities," O’Brien said. "I want to emphasize that."

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Police responded to a 911 call on Saturday morning from a person affiliated with the house where the overdose took place, according to NBC News.

Responding officers administered CPR and gave six of the victims doses of naloxone, an opioid overdose reversal medication.

All of the victims, between the ages of 19 and 30, appeared to know each other, O’Brien said.

“It certainly would have been far worse without the response and dispensing of naloxone by Chico police officers,” he added.

A narcotics task force is currently working to find the source of the fentanyl, which O’Brien said was likely combined with another substance. Fentanyl is often combined with heroin, and the pairing was found in one-third of fentanyl-related overdose deaths in 2016.

The Centers for Disease Control last month identified fentanyl as the deadliest drug in the U.S., as it was involved in 29 percent of deadly overdoses in 2016, up from just 4 percent in 2011. Increased use of the drug, which is often trafficked from China and Mexico, is linked to the opioid epidemic sweeping the U.S.