BTP officers are today releasing CCTV images after a gas was released on board a train carriage at Oxford Circus station.
The incident was reported at 9.13am on Saturday 20 July.
A number of people were treated at the scene by paramedics for coughing and lack of breath. There are no further concerns for their health.
Their symptoms would suggest the gas is CS gas.
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.