Police said a quantity of petrol had been recovered during searches of vehicles at the scene.
They added that searches of properties in London had been carried out, and investigative scenes around RAF Fairford were being scaled back and were expected to close completely by Wednesday.
Five men arrested on Sunday remain under investigation after being released on bail, with police saying the decision to release them was an investigative one
"I understand why this development will have come as a surprise to many but I'd like to reiterate that it was investigative decision, and one which is grounded in experience and careful consideration of policing powers," Laurence Taylor, head of national counter terrorism policing, said.
Taylor stressed that the suspects - all UK citizens in their 20s from London - face stringent conditions and "absolutely remain under investigation".
The five suspects are a 24-year-old from Camden, north London; a 23-year-old, two 24-year-olds and a 25-year-old all from Westminster, central London.
Police would not confirm when they are due to answer bail.
A police cordon remained in place around three vans found close to the Fairford base on Tuesday as armed police and counterterrorism officers continued to examine the scene.
A local farmer said she called police after seeing the vans blocking a road near the air base as she drove home in the early hours of Sunday.
She said she came across a group of men, some with their faces covered, nearby, and that the men ran away when they saw her vehicle.
Earlier, army bomb-disposal experts used a robot to examine the vans.
The air base houses the headquarters of the US air force's 501st Combat Support Wing and is a forward operating location for the US air force Global Strike Command.
Fairford was used during the 1990 Gulf War, the 2003 Iraq war and the US-Israeli bombardment of Iran this year.
It was unclear whether the arrested men had been on the security services' radars.
Local residents described seeing heightened security around RAF Fairford in recent days, with heavy vehicles parked across entrance gates.
Villagers living near RAF Fairford in Gloucestershire, dozens of whom were moved out from their homes on Sunday morning, were slowly returning to normal on Tuesday.
with AP and PA
