British Transport Police officers have released CCTV images after a gas was released onboard a train carriage at Oxford Circus station.
The incident was reported at 9.13am on Saturday 20 July.
A spokesperson for the BTP confirmed that several people were treated at the scene by paramedics for coughing and lack of breath.
Police believe that it was CS Gas that was emitted into the carriage and they have also confirmed that there are no furthers concerns for the health of those passengers who were affected.
Police enquiries have shown the gas was used during a fight between a group on board the train.
Officers believe the people in the images may have information that could help their investigation.
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.