Police are looking for two male suspects after gas was released in a tube train carriage in central London.
A number of people were treated at the scene by paramedics at Oxford Circus station for coughing and lack of breath.
British Transport Police said their symptoms “would suggest the gas is CS gas” and there were no further concerns for their health.
Officers hunting for the perpetrators have released CCTV images of two young white men they want to trace after the incident, which was reported at 9.13am.
Michael Roberts, 26, was with his girlfriend on the tube carriage affected by the gas.
He told the BBC he had his T-shirt over his mouth because he could not breathe and his girlfriend was “spluttering”.
Roberts said the effects of the gas lasted about an hour and described the ordeal as ”pretty frightening”, adding the train was about half full at the time.
“Two guys stumbled into our carriage and all the people on our carriage moved naturally away from the door,” he said.
“That’s when everybody realised we couldn’t breathe properly.”
Roberts added he initially thought the men were drunk “because they were trying to frantically get into the carriage” while the train was moving northbound between Green Park and Oxford Circus.
A Transport for London spokeswoman said the train had been taken out of service and to a depot for quarantine.
Anyone who witnessed the incident, or who knows the identity of the men, is asked to contact BTP by texting 61016 or by calling 0800 40 50 40 quoting reference 171 of 20/07/19.
