ROSEMONT, Ill. » Investigators were looking into the release of a gas that sickened several hotel guests and forced thousands of people — many dressed as cartoon animals for a convention — to temporarily leave the building.

Although some participants at the Midwest FurFest convention thought the mass evacuation was just part of the fun, investigators were treating it as a criminal matter.

Nineteen people who became nauseous or dizzy were treated at local hospitals, and at least 18 were released shortly thereafter. Within hours, emergency workers decontaminated the Hyatt Regency O’Hare and allowed people back inside.

The Rosemont Public Safety Department said someone apparently intentionally left chlorine powder in a ninth-floor hotel stairway, causing the gas to spread.

While authorities conducted their investigation, organizers tried to assure the participants that the evacuation would not overshadow the FurFest event, in which attendees celebrate animals that are anthropomorphic — meaning they’ve been given human characteristics — through art, literature and performance. Many of costumed attendees refer to themselves as "furries."

"In walk all these people dressed like dogs and foxes," said Pieter Van Hiel, a 40-year-old technical writer from Hamilton, Canada, chuckling as he thought about the crowd being herded into the Donald E. Stephens Convention Center as it was hosting a dog show.

Kit McCreedy, a 28-year-old from Madison, Wisconsin, said he didn’t think the incident would cause further disruption at Midwest FurFest, which was in its final day.

"I think we’ll recover from this," said McCreedy, his fox tail swinging behind him as he headed back inside. "People are tired but they’re still full of energy."

Others said they did not have a clue as to why anyone would intentionally disrupt the convention that includes dance contests and panel discussions on making the costumes, with some quick to point out that the brightly colored outfits are made from fake fur and foam and not real fur.

"Nobody uses real fur," said Frederic Cesbron, a 35-year-old forklift operator who rode a plane to Chicago from his home in France. He attended the convention dressed head-to-toe in a fox outfit that he said cost him about $2,000 four years ago but would go for $3,000 today.

Attendees said they came for fun, but also for the spiritual and artistic aspects of the convention that have them celebrating animal characters from movies, TV shows, comic books and video games. Some also create their own characters and appreciate being in an atmosphere where nobody seems surprised or shocked by an elaborate, bright purple dragon.

"Everyone is from a different background," said Michael Lynch, a 25-year-old from Madison, Wisconsin, who, like his buddy, McCreedy, dressed as a fox. "Nobody judges anybody. It’s nice to come to a place like that."

Or, as Van Hiel put it, "It’s kind of weird, but it’s not weird here."