U.S. Embassy Issues Security Alert to Citizens in U.K. Amid Suspected Terror Plot

The U.S. embassy in London has issued a security alert for American citizens in the U.K. following a suspected terror plot near an RAF airbase used by the U.S. military, which led to the arrests of five men who have since been released on bail.

“U.S. citizens are encouraged to exercise caution and be mindful of personal security,” read the notice responding to the Sunday incident near RAF Fairford. “U.S.-associated facilities, gatherings, and organizations may be at heightened risk. U.S. citizens are advised to follow instructions of local authorities for their personal safety and security.”

The alert reminded Americans that the U.K. in April raised its national terrorism threat level to “severe” from “substantial,” a decision made after an antisemitic attack in London was declared a terrorist incident by police. The “severe” level, according to the Metropolitan Police, means “an attack is highly likely.”

The warning came after British police arrested five men on suspicion of explosives and terrorism offenses after they were detained near RAF Fairford in southwest England, an airbase used by the U.S. to strike Iran, in the early hours of Sunday. Officers made the arrests after receiving a tip-off that three "suspicious vehicles" appeared to be heading toward the direction of the base, which Iran’s Islamic Revolutionary Guard Corps (IRGC) previously said could be a target on account of its use by the U.S. military.

The U.K.’s Counter Terrorism Policing (CTP) confirmed in a statement to TIME that the five arrestees were British nationals in their 20s who live in London. They were all released on bail Monday afternoon, local time. "They are subject to stringent conditions on their movement and contact with others. They absolutely remain under investigation as we explore multiple lines of enquiry," according to the CTP.

British Defense Secretary Wes Streeting told BBC Radio 4’s Today programme early Monday that it is too soon to speculate over the motives during a live investigation. Questioned about a potential Iran link, he said: "We know that Iran and their proxies wish to do us harm. We know that they hate our democracy, and we know that we regularly have to thwart Iranian attempts to threaten either our interests or indeed our allies."

CTP’s deputy assistant commissioner Vicki Evans said she was “very aware of the levels of speculation about the motivation behind this incident, and the geopolitical questions that are being raised,” but stressed more time is needed to work through the case.

Additionally, the head of CTP Laurence Taylor emphasized his officers are considering this case from "every possible angle," including the potential that this "may be activity committed by proxies, or individuals either knowingly or unknowingly, working on behalf of a foreign state."

The Iranian embassy in London has said it "categorically rejects and strongly condemns" any attempt to "link Iran to" the arrests made near RAF Fairford. "The Circulation of such fabricated speculations... will serve no purpose other than to fuel Iranophobia propaganda in the U.K.," it said Monday.

What we know about the arrests at RAF Fairford

RAF Fairford is a NATO-designated standby deployment and a U.S. forward operating base for U.S. Global Strike Command, according to U.K. government records. It has been used extensively by U.S. military aircraft in previous conflicts, including "Operation Iraqi Freedom" in 2003.

Gloucestershire Police, the local authority responsible for the area, received a call from a concerned resident at 12:45 a.m. local time on Sunday, regarding three suspicious vehicles traveling toward the airbase.

The local resident, a farmer, told police they had found the road blocked by three white vans. “I saw a large group of hooded and masked men. When they saw me, they ran ahead of me… I then realised that something highly unusual was taking place,” she later told the BBC.

Armed officers responded to the call and five men were arrested on suspicion of committing offences under the Explosives Act, as well as section 5 of the Terrorism Act.

Gloucestershire Police evacuated residents from around 85 nearby households and put a cordon in place while the British Army’s specialist explosive ordnance disposal team examined the vehicles. The incident is now being handled by CTP.

While the farmer who alerted police has been hailed for their actions, Streeting has indicated they only had a “partial picture” of the matter and appeared to suggest U.K. security forces had been aware of a suspected plot involving RAF Fairford.

U.S. President Donald Trump on Sunday claimed the suspects had been “under view for a long time.”

"Working with Britain, it was an amazing job. We had them under investigation. They were looking to do big damage to our fort, and working with the British worked out great… and we got them,” Trump told reporters.

When asked if Trump had been correct, Streeting did not reject the assertion.

“Our security services and our police, they don’t ask for praise, but I think it was a proud moment yesterday to see the President of the United States recognize the extraordinary work that our police and our security and intelligence services do every day throughout the year and throughout the world to keep us safe,” he said. “That’s all I want to say this morning on that.”

TIME has asked the Ministry of Defense and Downing Street whether the British authorities were already investigating a potential plot.

Speaking at the Labour Party conference later Monday morning, Streeting celebrated the U.K. and U.S. alliance as “the closest of any two nations on Earth” and vowed the two countries “will always stand together against those who hate our freedom and those who wish us harm.”

British Foreign Minister Ed Miliband, meanwhile, referred to the incident near RAF Fairford as a “reminder of the threats our country faces.”

The U.K. must be prepared to “confront” all the security threats facing Europe and the world, he told conference attendees, adding that “we will act against the proxies of Iran.”

Concerns over the risk Iran poses to the U.K.

The U.K. government gave authorization in March for the U.S. to use bombers based at RAF Fairford to carry ⁠out strikes on Iranian missile sites used to attack ships in the Strait of Hormuz. The base’s role in the U.S. military’s seven-month operation against Iran may have made the site a target.

While British police and lawmakers have said it's too early to determine who was behind the suspected terror plot, the U.K. government has repeatedly issued stern warnings regarding the threat the Iranian regime poses to the U.K.

On Feb. 28, former British Prime Minister Keir Starmer addressed the nation, explaining the U.K. had no involvement in the initial U.S.-Israeli strikes against Iran, but emphasizing the government’s belief that Iran cannot be allowed to possess a nuclear weapon.

“Even in the United Kingdom, the Iranian regime poses a direct threat to dissidents and the Jewish community,” he said. “Over the last year alone, they have backed more than 20 potentially lethal attacks on U.K. soil."

In March, four men were arrested on suspicion of conducting surveillance of locations and individuals linked to the Jewish community in London to assist Iran. One man was an Iranian national and the other three were British-Iranian nationals. The CTP said the arrests were “part of a long-running investigation and part of our ongoing work to disrupt malign activity where we suspect it.”

A separate investigation resulted in a 39-year-old man being charged in May with assisting a foreign intelligence service relating to Iran.

Earlier this month, a man and woman were arrested in London as part of National Security Act investigation. They were apprehended on suspicion of assisting a foreign intelligence service. Again, the country to which the investigation was linked was Iran.

The IRGC was designated a threat to national security in the U.K. under new powers in July, after a series of antisemitic attacks across England., including arson offenses against Jewish ambulance services.

“These new powers will make it easier to prosecute and lock up anyone carrying out [Iran’s] dirty work here in Britain,” said Starmer at the time.