Counter Terrorism Policing is leading an investigation after five men arrested near RAF Fairford were further arrested on suspicion of preparing a terrorist act.
Police said they received a call at around 12.45am on Sunday 27 September concerning three suspicious vehicles which appeared to be travelling towards the Gloucestershire airbase.
Armed officers responded and arrested five men in the Whelford area on suspicion of offences under the Explosives Act. They were subsequently further arrested on suspicion of preparation of a terrorist act, contrary to Section 5 of the Terrorism Act.
Counter Terrorism Policing takes lead
Counter Terrorism Policing has confirmed that it is now leading the investigation, working alongside Gloucestershire Police and other partners.
Deputy Assistant Commissioner Vicki Evans, Counter Terrorism Policing's Senior National Coordinator, said the investigation remained at an early stage and that all five men remained in custody.
No charges have been announced and an arrest on suspicion of an offence does not establish guilt.
Three vehicles reported travelling towards RAF Fairford
According to police, the incident began with a report concerning three suspicious vehicles which appeared to be travelling towards RAF Fairford.
Police have not publicly confirmed that the airbase was the intended target of any alleged activity, and investigators have not announced a motive.
RAF Fairford is used by the United States Air Force, giving the incident significant national-security interest, but any connection between the suspects' alleged intentions and the base itself remains part of the investigation.
85 households evacuated
A 400-metre cordon has been established around the vehicles while the Army's Explosive Ordnance Disposal specialists carry out examinations.
Around 85 households have been evacuated as a precaution while that work continues.
Police have said they believe the incident is contained and have asked the public not to speculate while evidence is gathered.
No motive or foreign-state link confirmed
At this stage, Counter Terrorism Policing has not publicly identified a suspected motive or attributed the incident to any terrorist organisation or foreign state.
Media reports have raised possible lines of inquiry, but these should not be treated as established facts unless confirmed by investigators.
The verified position is that five men remain in custody after being arrested on suspicion of Explosives Act offences and further arrested on suspicion of preparation of a terrorist act, with Counter Terrorism Policing now leading the investigation.