Police say a 'mass overdose' of the synthetic opioid fentanyl at a house in Chico, California, has left one man dead and 12 more hospitalized.

According to The Sacramento Bee, authorities arrived at a home on the 1100 block of Santana Court around 9am on Saturday after receiving a 911 call from someone inside.

'Upon arrival, Chico police officers found multiple individuals in what appeared to be life-threatening overdose conditions,' Chico Police Chief Michael O'Brien said at a news conference.

''Every indication - talking to medical staff, talking to doctors - everything is consistent with a fentanyl or fentanyl similar-type overdose.'

O'Brien said police performed CPR and administered six doses of naloxone, which is used to treat and reverse the effects of narcotic overdoses.

Police say a 'mass overdose' of the synthetic opioid fentanyl at a house in Chico, California, has left one man dead and 12 more hospitalized

Twelve people in the house received CPR and doses of naloxone, which is used to treat and reverse the effects of narcotic overdoses

An adult male was pronounced dead at the scene while 12 others - between ages 19 and 30 - were transported to Enloe Medical Center.

According to O'Brien, eight people were admitted, four of whom are listed as being in critical condition.

He added that it's possible the number of fatalities could rise.

The Bee reported that the first two responding officers were also taken to the hospital when they began feeling ill, potentially side effects of being exposed to fentanyl in the house.

Both were treated and released later in the day.

Officers have obtained a search warrant of the house, which is currently being treated as a 'hazmat site'.

Police suspect the overdose was caused by fentanyl and that tests are being run to confirm this belief.

'The amount of fentanyl - these substances are extraordinarily dangerous and it takes a very minute amount to cause [life-threatening] conditions,' O'Brien said.

In November, a new report from the Centers for Disease Control and Prevention (CDC) found that fentanyl was behind the increase of drug overdose deaths between 2016 and 2017.

Fentanyl is a synthetic opioid that is 100 times stronger than morphine.

The first two responding officers were taken to the hospital after the began feeling ill; they were treated and released later that day

According to the Defense Intelligence Agency, 68 percent of fentanyl and precursors used to make the drug originate in China

These factory-produced drugs, cheap and easy to make, are sold either directly to the US or via trafficking networks set up in Mexico.

According to The Bee, O'Brien says Chico officers began carrying naloxone in 2018 but, until recently, it has been used to reverse heroin overdoses.

He says he is concerned that fentanyl may become a problem in Chico.

'We were waiting, and have been waiting unfortunately, for this to happen in the sense that we knew fentanyl had been moving west,' O'Brien said.

'That is changing unfortunately, and now we've had this mass casualty incident... likely to have been cause by fentanyl.