Police are hunting two men who threatened a serviceman with a knife near an RAF base in Norfolk.
The serviceman, who was not in uniform at the time, is understood to have been approached by the men at RAF Marham at around 3pm on Wednesday.
The men then fled the scene in a dark coloured Ford Galaxy, police said.
It is unclear if the incident was terror related but Norfolk's police and crime commissioner said a "very intensive investigation" was underway.
A Metropolitan Police spokesman said: "The Met's counter-terrorism command is aware but the investigation remains with Norfolk Constabulary."
Police officers were carrying out reassurance patrols around the base into Wednesday night.
A Ministry of Defence spokesman said: "We are aware of an ongoing police operation in relation to an incident near RAF Marham. It would be inappropriate to comment further at this time."
The base is home to almost 10,000 people, including members of the RAF, Navy and Army, as well as the civil service and other staff.
A families day being held at RAF Marham on Thursday will go ahead as planned, a spokeswoman for the base said.