LONDON (AP) — British police arrested five men on Sunday on suspicion of preparing an act of terrorism and explosives offenses outside RAF Fairford, a military base in western England that’s operated by the U.S. Air Force.

Three suspicious vehicles are being searched, and counterterror officers are leading the investigation.

Here’s what to know about the base.

RAF Fairford, about 100 miles (160 kilometers) west of London, is one of several bases in Britain that has been used by the U.S. Air Force under an agreement dating from the 1950s. It is home to the headquarters of the U.S. Air Force’s 501st Combat Support Wing and is a forward operating location for U.S. Air Force Global Strike Command.

There are about 11,000 American military personnel stationed at various sites in Britain, according to the House of Commons Library. The number of U.S. personnel at RAF Fairford is not publicly available.

Fairford’s long runway makes it suitable for heavy bombers, and it has been used in conflicts including the Gulf War, the 2003 Iraq war and the U.S.-Israeli war on Iran this year.

Britain did not join the U.S.-Israeli war on Iran that began Feb. 28 and initially denied President Donald Trump’s request to use bases in the U.K. to strike Iran. But then-Prime Minister Keir Starmer announced days later that he had agreed to let the U.S. use bases in Britain for “defensive” operations against Iran’s missiles and their launch sites following Iranian attacks on U.K. interests and Britain’s allies in the Gulf.

American B-1 and B-52 bombers have taken off from Fairford for operations during the Iran conflict.

Anti-war campaigners have protested outside the perimeter fence of the base several times since the start of the Iran war.

It’s unclear whether the arrests are linked to the Iran war. British security officials say proxies linked to Iran are behind a string of arson and sabotage attempts in the U.K. and other countries, many of them targeting the Jewish community. They have previously warned that Tehran could expand its targets in the U.K. if conflicts in the Middle East continue.

Some local residents said security appeared to have been heightened around RAF Fairford in recent days, with heavy vehicles parked across entrance gates.

Another American base, RAF Lakenheath in eastern England, confirmed that it is on Charlie alert, the second-highest threat level that can be issued by the U.S. Department of Defense. According to U.S. military websites, Force Protection Condition Charlie “applies when an incident occurs or intelligence is received indicating some form of terrorist action targeting personnel or facilities is likely.”

There was no immediate confirmation of the threat level at RAF Fairford. U.S. European Command said it would not discuss “specific force protection measures.”

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