CHICO, Calif. (AP) — Police say a mass overdose at a house in Chico that killed one man and left four people in critical condition appears to have been largely caused by the dangerous opiate fentanyl, the Enterprise Record reports.

Chico police Chief Mike O’Brien said at a news conference that “every indication is that this mass overdose incident was caused from the ingestion of some form of fentanyl in combination with another substance. That is yet to be confirmed, but we do anticipate confirmation in the coming days.”

The newspaper says that a dozen people were taken out of the house Saturday morning and brought to hospitals.

Chico police commander Mike Rodden tells the Enterprise-Record that all of the people hospitalized were over 18 and most appeared to be in their 20s.

Steve Standridge, chief of Chico’s fire department, says cardiopulmonary resuscitation was performed on six people at the scene and a total of 12 were taken to the hospital.

Chico is a city of about 92,000 people about 160 miles north of San Francisco.