Sounds like they were doing something with fentanyl or an analog which maybe caused it to aerosolize or something, affecting most everyone in the house. Waiting to hear what really happened here:

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DEVELOPING - 1 Dead and 14 Treated for Drug Exposure After Mass Casualty Incident in North Chico
Updated 5:25 p.m. Saturday, Jan. 12, 2018 - Enloe Medical

Center confirms that they still have nine people in their hospital resulting from the mass overdose incident that occurred in Chico on Saturday morning.

Four people are in critical condition. Four others are in serious condition. One person is reported in fair condition and two others have been treated and released.

According to Oroville Hospital three patients were taken there for treatment. Two of them were police officers who have since been released. The condition of the third person taken to Oroville Hospital is not being released at this time.

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Updated 4:45 p.m. from the press conference held by Chico Police Chief Michael O'Brien:

Two of the responding police officers reported feeling the effects of a drug similar to Fentanyl, said Chico Police Chief Mike O'Brien at a press conference Saturday afternoon. They were some of the responders to a mass casualty incident on Santana Court in North Chico. One man died at the scene and 14 others were treated and taken to local hospitals for exposure to Fentanyl or a similar type of drug.

The officers have been treated, released, and have now returned to their homes for rest and further recovery, said Chief O'Brien.

The twelve people who were in the residence and were taken to the hospital were between 19-30 years of age, approximately, said Chief O'Brien. He said they were all adults. Four of them are in critical condition at this time. Chief O'Brien said more fatalities may occur due to this incident.

Four doses of Naloxone were used to help treat victims at the scene, O'Brien said.

He told reporters that someone at the residence, or associated with the residence called 9-1-1 a little after 9:00 a.m. Saturday morning to report the incident.

His detectives are continuing their investigation into what happened in the North Chico neighborhood near Ceres and East Avenue. The home on Santana Court is considered a Haz-Mat site, but neighboring homes are not, authorities said.

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https://www.actionnewsnow.com/content/news/1-Dead-and-14-at-Hospital-After-Drug-Overdose-in-North-Chico-Home-504268751.html