Two men have been re-arrested over an alleged plot near RAF Fairford, police said.
Counter Terrorism Policing re-arrested the two men on Tuesday as part of the investigation into activity near RAF Fairford in Gloucestershire last month.
The 24-year-old UK national and 25-year-old dual UK and Iranian national, both from the borough of Westminster, London, were arrested on suspicion of assisting a foreign intelligence service.
The country to which the investigation relates is Iran, police said.
The 24-year-old man was initially arrested on 27 September near RAF Fairford and the 25-year-old was later arrested on 1 October in Westminster, central London.
Five other UK nationals in their mid-20s were arrested after the incident, all of whom remain on police bail.
Assistant commissioner Laurence Taylor, head of national counter terrorism policing in the UK, said: "This is very much a live investigation, and our evidence gathering continues, but at this time our position remains the same; we consider it highly likely that there is Iranian state involvement.
"Our investigators are continuing to examine what that level of involvement was and the ambition behind it."
He added: "We know the public have unanswered questions. I want to be as transparent as possible and when we are in the position to share more, we will."
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Three suspicious vans were stopped near the airbase in Gloucestershire on 27 September.
Earlier this month, the US removed its bombers from RAF Fairford, partly because of a reported threat of a possible Iranian drone attack, American officials told NBC.
US President Donald Trump later told reporters outside the White House that "threats" led to the decision to remove the US bombers, including B-1s, B-2s and B-52s.
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