ROSLYN — A dozen young people — most of them women — were taken to hospitals after overdosing on an unknown substance that may have been slipped into their drinks at a house party in central Washington, police said Saturday.

All but one had been released by late Saturday afternoon, and that person was in good condition at a hospital in Ellensburg, according to police and a nursing supervisor.

The investigation started late Friday when officers and Kittitas County sheriff’s deputies received a report of a possible overdose victim at a Roslyn grocery store. Acting on information from the victim’s friends, police went to the house party and found 11 more young people either passed out or semiconscious.

Police believe something may have been slipped into their drinks. They expect toxicology tests on blood and urine samples taken from the victims to help determine what that might have been, said Cle Elum Police Chief Scott Ferguson.

“We don’t believe this was just a result of heavy drinking,” Ferguson said.

The emergency response required ambulances from all over the county.

When police began sweeping the house for possible victims, one sheriff’s deputy found and arrested a male who was engaging in sexual activity with a semiconscious female, Ferguson said. That person was released after investigators determined the woman to be his girlfriend, but Ferguson said police still have serious concerns about the incident and are still investigating.

Most of those at the party were students at Central Washington University.

Some students who attended the party told KOMO-TV of Seattle on Saturday that they believe a bottle of vodka had been spiked with the date-rape drug Rohypnol, commonly known as “roofies.”

“When I got there people started falling like flies,” said Chris Unger, a freshman. “I carried about four people downstairs. … Even if you’re a lightweight, you don’t just black out like that. We knew it was roofies.”

Another freshman, Katelynn Allen, called it scary.

“Everything was going fine, the music was playing, people were having fun — and then all of a sudden all the girls were puking everywhere,” she told the station. “Girls were outside on their backs.”

The students said several people at the party used vodka from the bottle to make mixed drinks. Those who brought their own alcohol, drank beer or didn’t drink any alcohol were not affected, the students said.

Cle Elum Sgt. Monty Moore said investigators wouldn’t know for sure until toxicology results come back.

“Right now we have no way of knowing for sure what drug was ingested by these folks,” he said.

College police, Cle Elum police and sheriff’s deputies were continuing the investigation.

The party coincided with Roslyn Crawl, an event featuring 20 bands at three venues. Roslyn, a former mining town with a population of about 1,000, is where the television sitcom “Northern Exposure” was filmed.