UK police said they had released five men held over an alleged terror plot against an airbase on bail, adding that they remained under investigation, including over possible links to a foreign state.

The men were detained early Sunday after three vans were spotted in the countryside near RAF Fairford, a UK-owned base that has been used by American bombers during decades of conflicts in West Asia.

Investigators haven't concluded whether the incident was state-backed. They are investigating a possible link to Iran, Russia or non-state militant groups.

The five suspects, who haven't been named, were initially held over suspected explosives offenses, and later also arrested on suspicion of preparing an act of terrorism. Police said the men are all British nationals from London and in their 20s.

The head of UK Counter Terrorism Policing, Laurence Taylor, said that the suspects face stringent bail conditions and "absolutely remain under investigation."

Taylor said lines of inquiry include that the suspected offenses were committed by "proxies or individuals, either knowingly or unknowingly, working on behalf of a foreign state."

Prime minister Andy Burnham said he would chair a meeting of the government's COBRA emergency committee later on Monday to discuss developments.

Residents were evacuated from 85 homes in the village of Whelford near the base on Sunday, and army bomb-disposal experts used a robot to examine three white vans.

Home secretary Shabana Mahmood said Monday that "thanks to the excellent work of the army's explosive ordnance disposal team," the scene was now safe and residents could go home.