Police are hunting for two men after gas was released on a Tube train in central London on Saturday morning.
A number of people were treated at the scene at Oxford Circus by paramedics for coughing and lack of breath.
British Transport Police said there were no further concerns for their health and their symptoms "would suggest the gas is CS gas".
Twitter users reported that the gas was released in a Victoria Line carriage, as the train approached Oxford Circus station.
CCTV images have been released of two young white men officers want to trace after the incident, which was reported at 9.13am.