No evidence has been made public yet.
That's everything we know.
Why this mattersSo why does this base matter so much?
RAF Fairford sits in Gloucestershire, about 90 miles west of London.
Since March, American B-1 and B-2 bombers have flown missions against Iran straight out of that base. Britain's government at the time called it defensive use.
Iran doesn't see it that way.
Back in July, Iran's Revolutionary Guard said any base used for aggression against Iranian territory is a legitimate target.
So when five men turn up near that exact base with petrol in their vans, the question isn't just what they planned. It's who sent them.
Britain's own police are asking that question too. Laurence Taylor, the assistant commissioner running the investigation, says officers are checking if the men were proxies, working for a foreign state, knowingly or not.
America got there first. Marco Rubio, the US Secretary of State, said days ago that this clearly involves the hands of a foreign actor. Donald Trump went further. He said the men had planned big damage, and said he wouldn't have let them out on bail.
Britain held back from naming Iran. Until now.
Ed Miliband, the Foreign Secretary, told his party's conference in Liverpool three days ago that Britain will act against the proxies of Iran. He didn't say Iran was behind this plot.
Andy Burnham has now gone further than all of them.
Iran's embassy in London had already denied any link, three days before Andy Burnham spoke. It called the coverage unfounded and malicious, warned of what it called Iranophobia, and said it reserves the right to take legal action.
That denial came before the Prime Minister's remarks. Abbas Araghchi's reply is the first one aimed straight at Burnham. And it does more than deny it. It turns Britain's own decision to bail the five men into evidence against Britain's case.
So now both governments are on the record, by name. Britain's Prime Minister says Iran was involved. Iran's Foreign Minister says Britain is chasing the wrong target.
Neither side has shown evidence. Nobody has been charged.
And what actually gave Britain its strong indications still hasn't been said out loud.
