A relationship between the victims has not been established, though police say they all knew each other to some extent.
One man has died and four are in critical condition following apparent drug overdoses at a house in Chico, the Enterprise-Record newspaper reports. "It certainly would have been far worse without the response and dispensing of naloxone by Chico police officers", O'Brien said.
Two police officers, the first to appear on the scene, were also taken to the hospital after reporting symptoms. O' Brien said the ages of the victims range from 19 to at least 30.
Chico Police Department did not immediately respond to Newsweek's request for comment. A total of 12 people were taken to the hospital. Authorities suspect the overdoses were caused by ingestion of the risky drug and a separate substance that is yet to be determined.
'The amount of fentanyl - these substances are extraordinarily risky and it takes a very minute amount to cause [life-threatening] conditions, ' O'Brien said.
Meanwhile, it's not clear where the Fentanyl originated and if there are more batches being sold around Chico, O'Brien said.
O'Brien said a narcotics task force is investigating the origin of the fentanyl, which may have been mixed with other substances.
O'Brien added that while not much fentanyl has been found in Chico in the past, emergency responders in the city have been "waiting" for the drug to arrive in the city.
The opioid epidemic has spread across the country in recent years.
"It was a large, mass casualty incident for us", the fire chief said in a news conference Saturday night.
"That should concern us all", he said.
"It is outrageous that Poisonous Synthetic Heroin Fentanyl comes pouring into the U.S. Postal System from China", he wrote on August 20. A report issued by the Centers for Disease Control in December showed fentanyl was the leading cause of overdose deaths in the US for 2016, the most recent year numbers have been released.
Fentanyl is "50 to 100 times more potent than morphine", according to the Centers for Disease Control.
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