ROSEMONT, Ill. — Chlorine gas sickened several people and forced the evacuation of thousands of guests from a suburban Chicago hotel early Sunday, including many dressed in cartoonish animal costumes for an annual furries convention who were ushered across the street to a convention center that was hosting a dog show.

Nineteen people who became nauseous or dizzy were treated at local hospitals, and at least 18 were released shortly thereafter.

The source of the gas was apparently chlorine powder left in a ninth-floor stairwell at the Hyatt Regency O’Hare, according to the Rosemont Public Safety Department.

Investigators believe the gas was created intentionally and are treating it as a criminal matter.

The hotel is hosting the 2014 Midwest FurFest convention, also called “Anthrocon,” where attendees celebrate animals that are anthropomorphic — meaning they’ve been given human characteristics — through art, literature and performance. Many of the attendees, who refer to themselves as “furries,” wore cartoonish animal outfits.

They said they come for fun, but also for the spiritual and artistic aspects of the convention, during which they celebrate animal characters from movies, TV shows, comic books, video games and characters they create themselves. Anthrocon conventions typically draw thousands of participants from around the world, many dressed in costumes that they say can cost as much as $3,000.

Guests were allowed to return a few hours later after the hotel had been decontaminated, and by mid-morning the furries were pouring back into the hotel for more activities, to chat with each other and to make their way to an outdoor courtyard where they took part in a group exercise session, with foxes, dragons and other characters getting an aerobic workout.

“We ask you to continue to be patient, and remember that the volunteers who make Midwest FurFest happen intend to give 110 percent to make sure that the fun, friendship, and good times … overshadow last night’s unfortunate incident,” organizers said in a statement posted on the group’s website. Organizers declined to discuss the matter in person.

Pieter Van Hiel, a 40-year-old technical writer from Hamilton, Canada, said the conventions are about having fun with people who enjoy the same hobby.

“This is an intense weekend of socialization. It’s kind of weird, but it’s not weird here,” said Van Hiel, who said he writes role-playing games for animals. He laughed as he described being herded out of the hotel and across the street to the Donald E. Stephens Convention Center: “There was a dog-grooming trade show going on and in walk all these people dressed like dogs and foxes.”

The conventions includes displays and vendors, an artists’ show, cartoon and character-related presentations, dances and live performances, and draw artists, puppeteers, costume makers, writers and just plain fans.

“Everyone is from a different background,” said Michael Lynch, a 25-year-old computer technician from Madison, Wisconsin. “Nobody judges anybody. It’s nice to come to a place like that.”

Dressed head-to-toe in a fox outfit, 35-year-old forklift operator Frederic Cesbron, of France, wanted to make one thing clear: “Nobody uses real fur.”