Thousands of people at the Hyatt hotel in Chicago – the site of a convention of 'furries' – were evacuated on Sunday, after chlorine powder was deliberately spilled in a stairwell.
At approximately 12.40am (4.40pm GMT) a 'noxious gas' was reported to be spreading across the ninth floor of the hotel in Rosemont.
Nineteen people were taken to hospital after complaining of nausea, dizziness and other problems, according to a statement by Rosemont Public Safety Department.
Local hospitals confirmed that patients had been treated for inhaling fumes from chemicals.
Investigators are reported to have discovered chlorine powder that was spilled on the stairwell of the ninth floor, in a manner that "suggests an intentional act" according to the report.
No one has claimed responsibility for sprinkling the chlorine powder in the Hyatt hotel.
Police are investigating the incident and are treating it as a criminal matter.
The area was decontaminated by emergency workers in Hazmat suits, and guests were allowed back into the hotel at approximately 3.30am local time.
Many of those ordered out of the hotel were attending the Midwest FurFest convention, and were dressed as cartoon animals.
The FurFest convention sees lovers of anthropomorphic cartoon animals dress up in life-sized costumes – 'furries' – "come together to celebrate… art, literature and performance based around anthropomorphic animals," according to the convention's website.
YouTube user Axio Wolf filmed a series of live streams outside the hotel, as the incident progressed.
Exposure to chlorine gas irritates victims' eyes and airways, causing difficulty with breathing and eyes to tear up. At higher levels exposure can lead to vomiting, airways swelling, muscle weakness and the victim to begin coughing blood, due to damage to lungs.
Chlorine gas was used as a chemical weapon in World War One. Its use during warfare was banned under the Geneva Convention, but the Organisation for the Prohibition of Chemical Weapons confirmed the gas was used to attack three villages in Syria earlier this year.