FAIRFORD, England — British police were investigating a possible terror plot near RAF Fairford on Monday after arresting five men and finding suspected explosives in three vans close to the U.S.-operated air base. Authorities have not determined whether the incident was directed by a state or linked to Iran, Russia or militant groups. The men were detained early Sunday after police received a late-night report of suspicious vehicles near the base, which is owned by the United Kingdom and has been used by U.S. bombers during the conflict involving Iran. The suspects, whose names have not been released, were first arrested on suspicion of explosives offenses and later on suspicion of preparing a terrorist act. Police may hold them for up to 14 days before charging or releasing them. Residents of 85 homes in nearby Whelford were evacuated Sunday. Army bomb-disposal specialists used a robot to examine the vans; as of Monday morning, residents had not returned and the vehicles remained at the scene. The accounts of how the arrests came about have differed. President Donald Trump said the suspects had been monitored for some time and that U.S.-British cooperation led to their detention. Police said the immediate trigger was a call about suspicious vehicles. British Defense Secretary Wes Streeting declined to discuss operational details, saying the government would share information when it could. The investigation comes amid heightened concern in Britain about activity linked to hostile states. Officials said in March that national-security cases involving hostile-state activity, including activity linked to Iran, rose 50% in the six months through December 2025. British security officials have also said Iran-linked proxies have been connected to arson and sabotage attempts in Britain and elsewhere in Europe. RAF Fairford, about 100 miles west of London, has taken on added significance during the Iran conflict. Britain initially refused Trump’s request to use U.K. bases for strikes, but Prime Minister Keir Starmer agreed in March to allow U.S. operations he described as defensive, aimed at Iranian missiles and their launch sites. U.S. B-1 and B-52 bombers have flown from Fairford during the conflict. Iran has said bases used to attack its territory could be considered legitimate targets. Foreign Secretary Ed Miliband called the incident a reminder of the threats facing Britain. The U.S. Air Force said personnel at Fairford remained vigilant but would not discuss security measures. The U.S. Embassy in London has urged Americans in Britain to be attentive to their personal security.