What do we know about the suspected Iranian plot to attack UK's Fairford airbase? - explainer

The United States has removed its fleet of bombers from a British airbase a week after five men were arrested nearby on suspicion of preparing terrorist acts as part of a suspected Iranian plot, the details of which remain largely unexplained.

Below is what we know so far:

What happened?

Police were called to RAF Fairford, 90 miles (145 km) west of London, early on Sunday, September 27, and arrested five suspects who were released on bail a day later.

A local farmer had called police, saying she saw three large white vans parked near the base and a group of "hooded and masked men" running into a field nearby. The Times reported that another person who contacted the police was one of the five arrested.

Armed officers responded to the calls and made the arrests in the Whelford area, about a mile from the base, before evacuating nearby properties and searching three vehicles.

The five were initially arrested on suspicion of offenses under the Explosives Act and later on suspicion of preparation of a terrorist act.

What happened next?

US President Donald Trump said those involved in the plot had planned "big damage," while US Secretary of State Marco Rubio also pointed to the "hands of a foreign actor."

In July, Iran's Revolutionary Guards had said the base was a legitimate target.

British counter-terrorism police later confirmed they were considering whether a foreign state had been involved, before Prime Minister Andy Burnham announced on Wednesday there might have been an Iranian link.

A 25-year-old dual UK-Iranian national was then arrested in London on Thursday, in what police called a "hugely complex" investigation.

A week after the initial arrests, a Pentagon official said all US bombers deployed to RAF Fairford had returned to their home stations in the US, without giving a reason.

A NATO military source said on Monday: "It is still believed that Iran poses an acute threat to US assets in the UK."

What has Iran said?

Iran has dismissed accusations that it was involved and on Thursday summoned the British ambassador in Tehran over the issue.

What about those arrested?

Counter-terror police have said the investigation is complex. They have said no explosive devices had been found as part of the investigation, but that a quantity of petrol had been recovered during searches of the vehicles.

The five men detained were all British nationals aged between 23 and 25 who lived in London, police said, before they announced they had released them all on bail.

The dual UK-Iranian national arrested on Thursday was also later released on bail.

They remain under investigation and are subject to stringent conditions on their movement and contact with others.

Trump said he would not have released the suspects on bail.

Why would Fairford be a target?

Fairford, which opened in 1944 and was used by Allied troops for the D-Day invasion of Normandy, has this year been used by US forces targeting Iran after Britain approved a US request for the base to be used for defensive strikes.

It is one of the US Air Force's only European airfields suitable for heavy bombers and is the preferred base for such operations. It played a key role in both the first Gulf War in 1991 and the invasion of Iraq in 2003.

Britain said in July its armed forces were ​ready to protect Fairford after Iran's Revolutionary Guards warned it not to allow US bombers to ‌fly from the base. The Guards said any base used to launch attacks would be a legitimate target.

The bombers deployed there were big enough to carry the large precision-guided "bunker-busting" weapons which are designed to penetrate fortified weapons stores.