One person is dead and a dozen others were rushed to the hospital after an apparent mass fentanyl overdose in Chico, California, on Saturday morning.
Four of those taken to the hospital are in critical condition, according to authorities. Police called the 13 people "friends and acquaintances," but did not provide exact relationships to the homeowner. The victims ranged in age from 19 to "about 30," police said.
Police did not provide many details of the person who died, but said he was an adult male who died on the scene.
Two police officers, the first to appear on the scene, were also taken to the hospital after reporting symptoms. They were treated and released, police said.
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.
"As tragic as this event is, and certainly there is potential for additional fatalities -- I want to emphasize that -- it certainly would have been far worse without the response and dispensing of Naloxone by Chico police officers, the life-saving efforts of Chico firefighters and Butte EMS and the emergency care of course received by Enloe Hospital staff," Chico Police Chief Mike O'Brien said.
Six doses of Naloxone, a drug which can reverse the symptoms of an opioid overdose, were given, according to O'Brien.
A mass casualty incident was declared to prepare Enloe Hospital for a large number of patients and every Butte County ambulance responded to the house, according to Chico Fire Chief Steve Standridge.
Chico police officers only began carrying Naloxone last year, O'Brien said.
"We do respond to many, many overdoses and particularly with the opioid crisis that is occurring everywhere in this country, to include Chico, California, we want to have in the hands of officers, who are often the first to arrive on scene, the ability to administer a life-saving drug," he said.
The mass overdose took place at one house in the city about 90 minutes north of Sacramento. That home is now being treated as a haz mat site and no one is being allowed to enter without proper precautions.
"Remember, the amount of carfentanil -- these substances are extremely dangerous -- and it takes just a very minute amount to cause life-saving conditions," O'Brien said.
The opioid epidemic has spread across the country in recent years. A report issued by the Centers for Disease Control in December showed fentanyl was the leading cause of overdose deaths in the U.S. for 2016, the most recent year numbers have been released.
Fentanyl, which is 30 to 50 times stronger than heroin, addiction specialist Dr. Nicholas Kardaras told ABC News last month, was mentioned in approximately 4,223 overdose deaths in 2014, 8,251 overdose deaths in 2015 and 18,335 overdose deaths in 2016.
"We were waiting and have been waiting, unfortunately, for this to happen in the sense that we knew fentanyl had been moving west and in other parts of the country they're really seeing the greatest impact of this drug," O'Brien said. "That is changing, unfortunately, and now we've had this mass casualty incident. ... That should concern us all."
There were 4.9 opioid overdose deaths per 100,000 people in California in 2016, according to the National Institute on Drug Abuse -- well below the national average of 13.3 per 100,000. The states with the highest overdose rates are mostly in the eastern U.S., including West Virginia, New Hampshire and Ohio, which take the top three spots.
In a related incident, ABC's Redding, California, affiliate KRCR reported their correspondent at the scene, Meaghan Mackey, was attacked by someone outside the house where the overdoses took place. The station reported Mackey was "very shaken up, but is okay" and police managed to stop the attack.
ABC News' Meghan Keneally contributed to this report.