COMMERCE CITY, Colo. — Authorities on Monday were investigating the deaths of five people found inside a Colorado apartment along with substances described as "illicit narcotics." Another adult and an infant were alive inside the unit.

Police officers responded to a call of an unconscious person at an apartment in Commerce City, about 7 miles northeast of Denver, on Sunday afternoon, police Chief Clint Nichols said. Officers found six adults and an infant inside the apartment. Three women and two men were dead and another adult and the infant, who was about four months old, were taken to a hospital, he said.

Police did not find evidence of trauma or violence and firefighters did not detect any hazardous gases inside, Nichols said.

The 17th Judicial District Attorney, Brian Mason, said Monday that it is likely the five people unintentionally overdosed on fentanyl, dying almost instantly. Preliminary testing and other evidence suggests that they believed they were taking cocaine and did not realize the drug had been laced with fentanyl, Mason told The Denver Post.

"And that is the danger of all drugs right now," he said. "No drug is safe. Because any drug could literally have fentanyl in it without the user knowing."

Fentanyl is an unpredictable and powerful synthetic painkiller that is blamed for driving an increase in fatal drug overdoses.

A police spokesperson did not immediately answer a request to respond to Mason's comments.

Nichols said that he did not know who the victims were but that some of them lived inside the unit.

The infant appeared to be doing fine, Nichols said. He said it was unclear if the baby's parents were among the people found dead in the apartment.

"I hope the parents were not in there, but I've been doing this long enough ... it would probably be safe to suggest the parents probably were inside and so for the infant, that's going to be a long time without parents," Nichols said.

He said the adult was "lucid" and talking to police.

The names and ages of those who died were not immediately available.