The United States has withdrawn all of its B-1 bombers from RAF Fairford, a British military base in Gloucestershire, following a security incident that prompted a counter-terrorism investigation.

A Pentagon spokesperson confirmed on Sunday that all US bombers deployed to the base had returned to their home stations in the United States. Roughly a dozen such aircraft had been based at RAF Fairford prior to the withdrawal.

"While operational security precluded us from confirming the movement of our assets and forces in real-time, we can acknowledge now that all US bombers that were deployed to RAF Fairford have re-deployed to their home stations in the United States," the spokesperson said.

The US did not officially state a reason for the withdrawal. However, Whitehall sources have not disputed that the decision was made in response to a security incident on 27 September. Sources also indicated it was a military rather than a political decision, taken at an operational level and communicated from the US military to its British counterparts.

On 27 September, police were alerted to three suspicious vehicles reported to be travelling towards the base. Five men, aged between 23 and 25 and all UK nationals from London, were arrested in the early hours of the morning on suspicion of explosives offences and, later, on suspicion of preparing a terrorist act. Investigators found petrol but no explosive devices in their vehicles. All five were subsequently released on bail in what police described as an "investigative decision" that was "grounded in experience and strategy".

A sixth man, a 25-year-old UK-Iranian dual national, was arrested in London on Thursday on suspicion of preparation of terrorist acts. Counter-terrorism police confirmed on Saturday that he had also been released on bail.

"The investigation into circumstances surrounding events in Gloucestershire is hugely complex, and our specialist teams are interrogating multiple lines of enquiry," said Vicki Evans, Senior National Coordinator for Counter Terrorism Policing. "As we've made clear, we're looking at all possible angles — including possible foreign state involvement."

UK Prime Minister Andy Burnham said there were "strong indications" that Iran had played a part in the incident but did not provide further details. Iran has strongly denied any involvement and summoned the UK ambassador to Tehran in protest over the arrests.

US Secretary of State Marco Rubio said the incident "clearly involve[d] the hands of a foreign actor" but declined to elaborate, saying he was limited in what he could say publicly. US President Donald Trump said he was "surprised" the five men had been released on bail.

"We know everything about them, we would not have released them," Trump said, adding that the UK had worked with the US "very closely" on the matter. Trump had earlier praised UK authorities for their efforts, saying: "The arrest in the UK was fantastic. Working with Britain — it was an amazing job. We had them under investigation. We had them under view for a long time, and we got them."

RAF Fairford, located approximately 160 kilometres west of London, has served as a forward operating location for the US Air Force for decades. In recent months, US bombers have taken off from the base for operations in the Middle East, after the UK approved a US request to use it for defensive strikes on Iran. The UK granted the US access to RAF Fairford and RAF Diego Garcia for defensive purposes in March.

In July, Iran warned that "any base used for aggression against Iranian territory constitutes a legitimate target for our forces."

The UK's Ministry of Defence has not commented on the withdrawal.