A transatlantic mystery is capturing attention in the U.S. and Britain. The central question is whether Iran was behind some kind of plot to target a British air base that is vital to the U.S. Air Force. Alternatively, was the initial speculation about an incident that has seen five men arrested, and then released on police bail, overhyped? President Trump and Secretary of State Marco Rubio have weighed in on the matter, as have senior figures in British politics and policing. But the whole matter has become more and more shadowy, and questions proliferate. Here’s a guide to where things stand Tuesday evening EDT.

In the early hours of Sunday morning, British time, five men were arrested close to RAF Fairford, an air base about 90 miles west of London in the county of Gloucestershire. The men had three white vans. The vans have been widely reported to carry the logos and details of a fuel company. But the markings were apparently counterfeit, and the men have no connection to the real company.

Initially, the alarm was said to have been raised by a farmer who was startled to see men wearing ski masks in the dead of night and the three vans oddly arrayed. However, in a Tuesday update, the U.K.’s head of Counter Terrorism Policing, Laurence Taylor, suggested the call was just one factor amid “a range of information received.”

The arrests sparked instant speculation that there was a link to the U.S. war on Iran. In March, then-British Prime Minister Keir Starmer gave permission for the base to be used by U.S. planes headed to Iran, with the proviso that this applied only to “defensive” missions, such as targeting Iranian military facilities. The U.S. Air Force’s 501st Combat Support Wing says on its website that Fairford is “home to the 501st Combat Support Wing Headquarters and the 420th Air Base Squadron and serves as the preferred bomber forward operating location in Europe for USAF [U.S. Air Force] Global Strike Command.”

The use of bases for the U.S. war on Iran is controversial in Britain, where both Trump and the war are unpopular. Deepening intrigue. When news of the arrests first broke, the idea that a “terror plot” had been uncovered commanded the headlines. This could yet prove to be true. And it isn’t just conjecture out of thin air, either. In July, Iran had warned that any base “used for aggression against Iranian territory” would be considered a “legitimate target.”But things now seem far more complicated. The men were initially arrested on suspicion of explosives offenses.

But Taylor, the British counterterrorism chief, confirmed Tuesday that no explosives had been found in the three vans, though “a quantity of petrol was recovered.”It has been widely reported in the British media that the five men are all U.K. nationals in their early-to-mid 20s who live in London. This does not preclude a link to Iran, of course, but it seems a bit incongruous with the “terror plot” thesis. Adding to the perplexity is the British police’s decision to release the men on bail. It’s near-certain that bail would have been refused if the men were seen as a present danger to the public or to national security.

Taylor said Tuesday the men must “adhere to strict conditions” while on bail and emphasized the investigation is continuing. He added, regarding the granting of bail, “I understand why this development will have come as a surprise to many, but I’d like to reiterate that it was an investigative decision, and one which is grounded in experience and careful consideration of policing powers.”In the past few hours, there has been a new, bizarre wrinkle: The Times reported Tuesday evening U.K. time that the first call alerting authorities had actually come from one of the suspects.

The British newspaper said one of the men called 999 — the British equivalent of the 911 emergency line — almost one hour before the call from the farmer.

The Times is a well-regarded news organization, but nobody is sure what to make of this new detail. The president is among those startled by the decision to release the five men on bail. On Monday, Trump said he was “surprised” by the move. “I would have said don’t let them have bail,” he told reporters in the Oval Office. “I wouldn’t have done that.”Pushed on whether Iran was behind the episode, Trump offered an enigmatic answer: “I know the answer. I can be very specific. But I’d rather not say.”

Rubio, speaking with Fox News on Monday evening, called the events at RAF Fairford “a very serious situation” and added that it was “one that clearly involves the hands of a foreign actor.”The Trump and Rubio comments are clearly suggestive of Iranian involvement. While Trump often shoots from the hip, it is less likely that Rubio is making such a claim without some supporting evidence. But it’s not clear so far what that evidence might be.

Some confusion also surrounds comments Trump made at an earlier stage, soon after the arrests, when he said of the men, “We had them under view for a long time, and we got them.”British media have cited anonymous sources suggesting this may be inaccurate. It’s also possible that what Trump said was correct but that British authorities had wanted to keep surveillance or even the use of informants under wraps. The Iranian Embassy in London has emphatically denied that the Islamic Republic is behind the incident.

In a Monday social media post, the embassy called suggestions of a link “unfounded and malicious speculations.”The embassy also complained that such claims “will serve no purpose other than to fuel Iranophobia propaganda in the UK.”Everyone is trying to square the apparent contradictions. On one hand, there is the arrest of a group of men behaving suspiciously near a base used by U.S. forces — a location that Iran plainly considers a legitimate target.

On the other hand, there’s the lack of explosives, the decision to bail the men, and the fact that they have not been charged.

There is no actual proof of wrongdoing as of Tuesday evening, and the men are entitled to the presumption of innocence even if they were to be charged.

Among those who believe something nefarious was going on, the least grave suggestion is that the men could have been fuel thieves with no political motivation at all. But anonymous sources quoted in the U.K. media appeared to be dampening that speculation on Tuesday.

One theory that has got more traction is that the men could have been conducting a “dry run” aimed at testing security. If that were the case, could the emergency call from the suspect have been intended to gauge police reaction times?

But, if it was, why would the men have let themselves get arrested rather than fleeing before police arrived?

There has been talk in British security circles of Iran using proxies — often low-level criminals paid to carry out basic tasks. And it’s also possible that the men were engaging in a stunt to sow confusion and fear.

It’s worth reiterating it’s also possible the five men are innocent of any crime at all. For now, the case is captivating in part because nobody knows.