CCTV images have been released of a man police want to trace after teenagers were threatened with a knife on a Tube station platform.
A man attacked a fellow passenger on an eastbound train between Loughton and Theydon Bois at 8.30pm.
The incident was witnessed by a group of five 14-year-olds on the train, who alerted fellow passengers.
The teenagers then left the train at Theydon Bois, followed by the attacker, who then produced a flick knife and threatened the group before boarding the next train to Epping.
Police are reviewing more CCTV from the station and surrounding area and would like to hear from anyone with information about the incident, which happened on December 20.