The move follows the September security scare near the US airbase in Gloucestershire and comes as officers stress that Iranian state involvement remains highly likely.
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Counter-terrorism police have re-arrested two men in connection with the security incident at RAF Fairford last month, this time on suspicion of assisting a foreign intelligence service.
The arrests, announced on 20 October, mark a significant development in an investigation that has already seen seven men detained and released on bail.
A 24-year-old UK national from Westminster, who was among the five men first arrested near the Gloucestershire base on 27 September, and a 25-year-old dual UK-Iranian national, also from Westminster, who was arrested days later in London, are being held under Section 3 of the National Security Act 2023.
Police confirmed that the country to which the investigation relates is Iran.
Assistant Commissioner Laurence Taylor, head of Counter Terrorism Policing, described the inquiry as “hugely complex” and said evidence gathering continues.
He reiterated the assessment that Iranian state involvement is “highly likely.”
A warrant of further detention has been granted, allowing the two men to be held for up to seven days from the point of their latest arrest.
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What happened at Fairford
In the early hours of 27 September, police were alerted to three suspicious vans near RAF Fairford, a base used by the US Air Force.
Five men were arrested at the scene.
Petrol was recovered from the vehicles, but officers later stated that no improvised explosive devices were found.
The men were initially held on suspicion of offences under the Explosives Act and preparation of terrorist acts before being released on bail.
Two further arrests followed, bringing the total to seven. All were released under investigation.
The US subsequently withdrew its B-1 bombers from the base.
Prime Minister Andy Burnham has previously told Parliament that the activity was highly likely to have been coordinated in Iran and almost certainly directed by the Islamic Revolutionary Guard Corps.
Iran has denied any involvement.
Questions over bail and transparency
The re-arrests have intensified public and political scrutiny of the initial decision to release the suspects on bail.
Critics have questioned whether potential national-security risks were given sufficient weight, particularly given the sensitivity of the site and the subsequent assessment of Iranian links.
Police have maintained that the investigation is complex and that decisions are based on the evidence available at each stage.
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Official statements have been careful.
Officers have not claimed that viable bombs were present and have avoided detailed public commentary on the precise roles of each individual while inquiries continue.
Some of those originally arrested may not have been fully aware of any wider plan, according to police assessments still under review.
Wider context
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The Fairford episode sits against a backdrop of heightened concern about foreign-state threats, particularly from Iran, and the challenge of investigating potential proxies or facilitated activity inside the UK.
The National Security Act 2023 was designed to give police stronger tools against such activity.
The fact that two of the original suspects are now being held under its provisions for assisting a foreign intelligence service underscores the seriousness with which investigators are treating the Iranian dimension.
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At the same time, the case has fuelled broader political arguments about immigration, citizenship and security.
Some commentators have pointed to the dual nationality of one suspect and the London backgrounds of those involved to argue that long-term migration and integration failures create vulnerabilities that hostile states can exploit.
Others insist that citizenship and individual criminal responsibility, not ethnicity or immigration status, are the proper focus of any investigation, and that linking the incident to mass migration over decades risks oversimplifying a specific intelligence and criminal inquiry.
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Police have said they want to be as transparent as possible and will share more information when they are able.
Until charges are brought or the investigation concludes, many details remain under active examination.
The two men currently in custody face further questioning; the other five remain on bail.
The outcome will determine whether the Fairford incident is confirmed as an Iranian-directed operation or remains a more ambiguous security scare that nevertheless exposed significant gaps in real-time threat assessment.