British counterterrorism police have released five young men on bail during an investigation into a suspected bomb plot targeting a US-operated air force base in England. No explosives were found in three vans linked to the suspects, police confirmed on Tuesday.

The five men, all British nationals in their twenties, were arrested early on Sunday near RAF Fairford on suspicion of explosives and terrorism offences. They were freed a day later under stringent bail conditions, the head of UK Counter Terrorism Policing, Laurence Taylor, said.

The suspects ‘absolutely remain under investigation,’ Taylor said. He called the bail decision an investigative move ‘grounded in experience and strategy.’ US President Donald Trump said on Monday he was surprised by the release. ‘I wouldn’t have done that,’ he told reporters.

Secretary of State Marco Rubio told Fox News the case ‘clearly involves the hands of a foreign actor,’ without naming a country. Detectives are pursuing ‘multiple lines of inquiry,’ including whether the suspects had a link to Iran. Iran has denied involvement.

RAF Fairford has hosted US bombers during decades of conflicts in the Middle East, including this year’s war with Iran. The investigation continues.