The Metropolitan Police said the three white vans had their rear doors open and were positioned on a rural road near the base, and bomb disposal robots were dispatched within minutes of the 999 call.
RAF Fairford, home to the U.S. 501st Combat Support Wing, has been a key logistics hub for U.S. aircraft since 2013, and a spokesperson for the wing said no American personnel were involved in the incident.
President Donald Trump, speaking to reporters in Chicago, said the suspects had been "looking to do big damage" to the "fort" and that U.S. and UK intelligence had been working together.
The local community was rattled when 85 homes were evacuated, and a Whelford resident told the PA news agency that a neighbour had noticed the vans and called 999.
Prime Minister Andy Burnham has been briefed, and Defence Minister Wes Streeting said the UK is coordinating closely with U.S. officials, while the Metropolitan Police have declared a major incident and imposed a 400‑metre cordon.
The investigation is ongoing, with the UK police and U.S. intelligence expected to release further details in the coming days as they assess the threat.