FAIRFORD, England: The five suspects arrested and questioned by British police on suspicion of terrorism and explosives offenses were all British nationals from ​London in their early 20s, police said on September 28.

British officials said they were still investigating any possible motive for the incident near RAF Fairford.

The five men were detained near RAF Fairford, a military air base in western England that has been used by the United States for strikes against Iran, early on September 27 after police received information that three large white vans had been seen traveling towards the airfield. They have since been released on bail.

U.S. President Donald Trump said the suspects had been planning to cause "big damage" and had been under investigation by British and U.S. authorities. However, a local farmer said she had contacted police after seeing a group of hooded and masked men running into nearby fields.

British counterterrorism detectives have not publicly identified who they believe was behind the incident. A source familiar with the investigation said an Iran-linked motive was considered the most likely possibility. Police were also examining the possibility of a Russian sabotage operation or an Islamist plot.

Defense Minister Wes Streeting told BBC News on September 28 that he would not comment on the possible motive while the investigation was continuing.

He said Britain faced a broader threat from Iran and groups linked to it. "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."

In May, Britain imposed sanctions on 12 individuals and organizations linked to Iran. The government accused them of involvement in hostile activities, including planning attacks and providing financial services to groups seeking to destabilize Britain and other countries.

Iran has repeatedly denied involvement in attacks or plots in Britain and elsewhere.

The British government had authorized the United States to use bombers based at RAF Fairford to attack Iranian missile sites that were targeting ships in the Strait of Hormuz.

The government said Prime Minister Andy Burnham was being regularly updated about the incident. He also canceled his attendance to at least two events at the Labor Party's annual conference on September 27.

Iran's Revolutionary Guards warned Britain in July against allowing U.S. bombers to operate from Fairford. They said any base used to launch attacks would be considered a legitimate target.

Police established a 400-meter security cordon around the Fairford base and evacuated residents from 85 homes.

A bomb-disposal robot was seen operating near the three white vans on September 27, as security measures were increased around the base. Armed police guarded the main entrance, while heavy machinery blocked other access points.

Security was also increased at other U.S. military bases across Britain.