British authorities arrested two men on suspicion of assisting Iranian intelligence in a second set of charges for suspects after an alleged plot at RAF Fairford airbase last month.
Counter-Terrorism Policing said on Tuesday that the men would face a new charge under the National Security Act.
The US withdrew its B-1 bombers from Fairford after five British men were arrested outside the military base in south-west England in the early hours of September 27.
Two other men were later arrested by police, although all seven were released on bail.
A statement on Tuesday said authorities had re-arrested a British national, 24, who was initially arrested outside RAF Fairford, and British-Iranian dual national, 25, who had previously been arrested in London.
"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," assistant commissioner Laurence Taylor, head of National Counter Terrorism Policing in the UK, said in a statement.
British Prime Minister Andy Burnham last week said that the activity at Fairford was likely to have been co-ordinated in Iran, while Iran's ambassador to Britain was called in for a diplomatic rebuke. Iran rejects the allegations.
"We know the public have unanswered questions," Mr Taylor said. "I want to be as transparent as possible and when we are in the position to share more, we will."
US officials said they remain confident that no operational problems had been caused by returning the bombers from the UK, which is the only European country that allows in aircraft involved in the Washington's Iran operations.
"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," a Defence Department official said in a statement at the time.
Jonathan Hackett, a former US Marines interrogator, told a panel hosted by the Royal United Services Institute think tank this week that Iran's Islamic Revolutionary Guard Corps regarded the UK as a soft target for sabotage operations.
"Because the UK is one of the only allies and partners that's allowing the United States to land strategic bombers like the B-1B Lancer," Mr Hackett said.
"British international overseas territories like Diego Garcia, are the fallback location for those bombers. Other partners, Nato partners, are not allowing those assets to land. So the IRGC is looking at the UK as kind of an accomplice in this, and is targeting it as much as it can.
"It's a low capability but it's still a threat."