How did the RAF Fairford incident go from 'suspected bomb plot' to police bail?

On Sunday morning, the world was watching as drama unfolded in the village of Whelford.

Before sunrise, residents were called from their beds and told to evacuate as a huge police operation unfolded in the village, which is next door to RAF Fairford, a base used by the US Air Force.

One villager managed to take a picture of five men sitting on the ground, stripped to the waist, handcuffed and under armed guard.

They were arrested for offences under the Explosives Act. But now counter-terrorism detectives say no explosives were found in their three vans, only an "accelerant" - petrol.

What some newspapers called a "suspected bomb plot" ended in the suspects being bailed. So what was going on?

The US President Donald Trump asserted that the men had been trying to do "big damage" to the base. "We had them under view for a long time, and we got them," he declared.

US Secretary of State Marco Rubio said: "What could have happened in the UK over the weekend is a very serious situation. It's one that clearly involved the hands of a foreign actor."

But in a veiled criticism of that, the head Counter Terrorism Policing in the UK Laurence Taylor said that the worry that a lot of people had been experiencing had been "heightened by the international commentary".

He also defended his detectives' decision to give the suspects police bail with "very strict conditions". This had been criticised by President Trump.

Taylor said it was an "investigative decision and one which is grounded in experience and strategy".

This suggests there were good tactical reasons for releasing the men on bail, perhaps to encourage their cooperation.

In truth, once it became clear that the military remote-controlled robots that were used found no explosives in the vans, only petrol, the investigation inevitably changed.

It left detectives with five men from London in their 20s in police custody who had simply been found parked up near the air base in the middle of the night in suspicious circumstances and with some petrol - but little other usable concrete evidence.

It might have been hard to persuade a senior officer or a magistrate to extend the suspects' detention without something more definite.

The men were being held under the Police and Criminal Evidence Act (PACE), not the Terrorism Act (TACT). This meant detectives had the option to use police bail with strict conditions, but could not hold the men for 14 days, which they could have done if TACT had been used.

For a while it was unclear whether the men were already being watched by undercover surveillance teams when they were spotted by villagers in the dead of night, or whether the villagers really were the first people to alert the police.

Taylor has now given some assistance on this, saying the response on Sunday morning was "due to a range of information that we received", suggesting the police had some other intelligence

Certainly security had been visibly stepped up at the base in the preceding days, so certainly there had been some intelligence about a possible attack. Elite firearms officers were very quick to respond to the villagers' 999 calls.

Both counter-terrorism and military sources have told me that this story is still quite near the beginning, raising the possibility that detectives are trying to identify some kind of "controlling hand" behind the presence of the suspects in Whelford with their petrol.

That leaves a number of options open:

• The men could be entirely innocent and able to give a good explanation for being in Whelford on Sunday
• They could have been paid or tricked into moving the vehicles and the petrol, but unaware of who they were working for
• Or they could have been part of a plot which was interrupted before it reached a more dangerous phase

That is why the next bit of the counter-terrorism investigation is so vital.

By examining mobile phones and computers, tracking vehicles, searching addresses, and using the covert techniques of MI5, GCHQ and US intelligence, detectives are trying to build a clearer picture.

On that critical investigation rests the decision over whether to arrest anyone else, whether to charge the five men, and whether we ever find out the true story of the mysterious RAF Fairford plot.