Police are hunting for two men after gas was released on a Tube train in central London on Saturday.
A number of people were treated at the scene at Oxford Circus by paramedics for coughing and lack of breath.
British Transport Police there were no further concerns for their health and their symptoms "would suggest the gas is CS gas".
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.
The incident occurred at around 09:13 BST on board a Victoria line Tube.
A number of London Underground passengers were treated for coughing and shortness of breath at the scene.
British Transport Police has appealed for witnesses and released CCTV images of the two men they would like to speak to in connection with the investigation.
Passenger Michael Roberts told BBC he saw two people "looking into the next carriage had seen some sort of a commotion".
"Then two guys stumbled into our carriage and all the people on our carriage moved naturally away from the door.
"That's when everybody realised we couldn't breathe properly."
The train was taken out of service and transferred to a depot where it is in quarantine.