British police on Tuesday rearrested two men on suspicion of assisting Iran's intelligence service in connection with an alleged plot targeting a US-operated air base in England last month.
Counter Terrorism Policing said the suspects, a 24-year-old British national and a 25-year-old dual British-Iranian national, were arrested under the National Security Act.
One of the men was initially arrested on 27 September, the day the suspected plot targeting RAF Fairford took place, while the other was first arrested on 1 October. Both had been released on bail pending further investigation before being rearrested on Tuesday.
RAF Fairford, located in western England, has been used by US forces to launch bombing operations against Iran during this year's war.
The suspected incident prompted the evacuation of dozens of residents from homes near the base as armed police officers and military bomb-disposal experts searched the surrounding rural area.
Police later said they had discovered a quantity of petrol, an accelerant, in three vans at the site, but no explosives were found.
British authorities have linked the incident to Tehran, which has denied involvement. The suspected plot also prompted the United States to withdraw its bombers from RAF Fairford.
UK Prime Minister Andy Burnham said the alleged plot was "almost certainly" directed by Iran's Islamic Revolutionary Guard Corps (IRGC).
Iran has previously said that any military base used to launch attacks against its territory would be considered a legitimate target.
Five other men in their 20s were arrested earlier in connection with the investigation and subsequently released on bail pending further inquiries. No one has been charged so far.
Laurence Taylor, head of the UK's Counter Terrorism Policing, said detectives were pursuing multiple lines of inquiry in what he described as a highly complex investigation.
"This is very much a live investigation, and our evidence gathering continues, but at this time our position remains the same; we consider it highly likely that there is Iranian state involvement," he said in a statement.
Investigators are continuing to examine the extent of any Iranian involvement and the objectives behind the suspected plot, he added.
Last week, MI5 chief Ken McCallum warned that Iran and Russia were stepping up their hostile activities against the UK, saying the incident at RAF Fairford was not an isolated case.
McCallum said Tehran was seeking to expand its activities in Britain beyond its traditional targets, including Iranians opposed to the country's Islamic authorities.
More than 50 people have been arrested over Iran-linked plots in the UK over the past year, he said.