UK Prime Minister Andy Burnham said on the BBC that a complex investigation is underway to understand how five men and three vans came to the perimeter of RAF Fairford last weekend.
Police recovered a quantity of gasoline in the vans, but no explosives were found.
The men were released on bail the next day, a move that drew criticism from US officials, including President Trump and Secretary of State Rubio.
Iranian authorities denied any involvement, while Burnham said cooperation with US counterparts is ongoing.
RAF Fairford, 100 miles west of London, has hosted US bombers for decades, including during this year's conflict with Iran.
Investigators will continue probing the suspects' connections to foreign actors, with a full report expected by late April.