The reported release of a smoke bomb by football fans on a train is being treated as a criminal offence, British Transport Police (BTP) has said.
Footage appears to show Reading FC supporters chanting songs about the club and banging on the train carriage ceiling as it fills with smoke.
The incident occurred the day after a terrorist attack on a tube train in Parsons Green.
Reading FC said the "irresponsible behaviour" was "entirely unacceptable".
Footage was posted to Twitter on Saturday afternoon, seemingly before the Royals' 1-1 draw against Brentford.
A club spokesperson said that it had "actively assisted the relevant local authorities with an investigation" and appealed to fans to "report any information which may assist in the identification of those responsible to the British Transport Police".
The spokesperson added that the club "acknowledge the vast majority of the 1,650 away fans behaved impeccably in backing the team from the first whistle to the last at Griffin Park".
BTP has urged anyone with information about the incident, which took place as the UK's terror level was at its highest rating of "critical", to come forward.