A massive fentanyl overdose incident in California on Saturday left one person dead and sent a dozen others to the hospital, police said.

NBC News reported that the overdoses were caused after the individuals had been exposed to fentanyl, a deadly synthetic opioid.

One person connected to the people in the house where the fentanyl overdose occurred called 911 on Saturday morning. Officers who responded to the scene administered six doses of the opioid antidote naloxone in the hopes of reviving the victims.

The exposure was so powerful that two of the responding officers fell ill from fentanyl poisoning. Both received treatment at a hospital and were later released, police said.

Mike O’Brien, the captain of the Chico, California, police force, told reporters at a press conference that four of the hospitalized victims remain in critical condition.

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

O’Brien said all the victims, who are between the ages of 19 and 30, seem to know each other.

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

O’Brien said a narcotics task force is investigating the origin of the fentanyl, which may have been mixed with other substances.

The Centers for Disease Control and Prevention (CDC) released a report in December calling fentanyl the “deadliest drug in America,” surpassing drugs like cocaine and heroin.

Because of the drug’s harmful capabilities, some experts say that criminals could use the deadly synthetic opioid as a “weapon of mass destruction.”