Police released CCTV after a 'violent threat' was made to a mother and daughter on a bus journey from Brighton to Shoreham.
British Transport Police (BTP) said the incident took place between 6pm and 7pm on Sunday, December 23 .
The BTP spokesperson said: "A group of teenagers are reported to have been acting abusive to other passengers and shouting and swearing on board the service.
"When the mum and child got up to exit the bus, one said he would hurt them if they complained about the group’s behaviour.
"Officers believe the person in the image may have information which could help their investigation."
Anyone who knows the individual is asked to contact BTP by texting 61016 or by calling 0800 40 50 40 and quoting reference 43 of 24/12/18.