Police are hunting for two men after gas was released on a Tube train in central London on Saturday.
Some passengers were treated at the scene at Oxford Circus by paramedics for coughing and lack of breath.
Enquiries have shown the gas was used during a fight between a group on board the train.
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. They believe the people in the images may have information that could help their investigation.
Passenger Renae Brown wrote on Twitter: “Got off at Oxford Circus about 20 mins ago because a gas of some sort went through the carriage and made everyone start coughing/crying. We’re okay now.”
Michael Roberts, another passenger, tweeted: “Just before Oxford Circus some kind of non-smelling non-visible gas entered the carriage causing passengers to choke.”
Police are asking anyone who witnessed the incident or knows the identity of the men to contact British Transport Police by texting 61016 or by calling 0800 40 50 40 quoting reference 171 of 20/07/19.