"Unfortunately one male individual was pronounced dead at the scene", O'Brien said, adding that the residence where the overdose occurred is being treated as a hazmat site.
One person died, and 14 people were taken to the hospital.
"Certainly there's potential for additional fatalities", O'Brien said.
Eight of the victims were admitted, and of those eight, four were listed as being in critical condition.
Four of the victims were in critical condition, said Mike O'Brien, a police captain in Chico, California.
The mass overdose appears to have been largely caused by the unsafe opioid fentanyl, Chico police Chief Mike O'Brien said at a news conference.
Authorities said "everything was consistent" with a fentanyl overdose, but testing was still being done to confirm the drug was the cause of the death and injuries.
Both were treated and released later in the day.
Two officers who responded to the scene also went to a hospital after they said they felt the effects of a drug similar to fentanyl, KHSL-TV reported.
Six doses of the opioid antidote naloxone were administered to the victims, who are believed to be friends 19 to 30 years old.
Officers have obtained a search warrant of the house, which is now being treated as a "hazmat site".
The victims, O'Brien said, "generally" all "knew each other", according to the Bee.
"That is changing, unfortunately", He said, "and now we've had this mass casualty incident ... likely to have been caused by fentanyl".
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.
O'Brien said a narcotics task force is investigating the origin of the fentanyl, which may have been mixed with other substances.