A British-Iranian man has been arrested in connection with an alleged security incident at an airbase in southwest England over the weekend.

Counter Terrorism police in the UK said on Thursday that a “27-year-old man, a dual UK/Iranian national, was arrested” in central London.

He was detained “on suspicion of preparation of terrorist acts” as part of an investigation into the events on Sunday near the Royal Air Force (RAF) Fairford.

A second man, 26 – a UK national – was also “questioned under caution”, the police statement added.

The RAF base 160 kilometres (100 miles) west of London has been used by US bombers during decades of conflicts in the Middle East, including the US and Israel’s war on Iran, which began in February.

On Sunday, five men, all in their 20s and from London, were arrested on suspicion of explosives offences and “terrorism” over the incident at RAF Fairford. They have since been released on bail.

“The investigation into circumstances surrounding events in Gloucestershire is hugely complex, and our specialist teams are interrogating multiple lines of enquiry,” Vicki Evans, the senior national coordinator for Counter Terrorism Policing for the Metropolitan Police, said in a statement.

“As we’ve made clear, we’re looking at all possible angles – including possible foreign state involvement.”

Police on Tuesday said they had found no explosives in the vehicles that were parked near the base on Sunday, but had retrieved a “quantity of petrol”.

Earlier, UK Prime Minister Andy Burnham said there were “strong indications” that Iran played a role in an alleged security incident.

Iran, however, strongly denied any involvement in the incident and the country’s foreign minister, Abbas Araghchi, hit back at Burnham’s allegations, saying British authorities were “barking up the wrong tree”.

Iran’s embassy in London on Monday said the attempts to link Iran to the individuals arrested near the airbase were “fabricated speculations” that fuel “Iranophobia propaganda”.

Al Jazeera’s Rory Challands, reporting from London, said Thursday’s development marked the “sixth arrest” related to the incident, but apart from the nationality of the suspect, “until now nothing in this case has been made public that specifically links Iran to the incident”.

There continues to be lack of clarity around what happened on Sunday, Challands added.

“Much of this whole incident is shrouded in mystery, still. The nature of the threat has still not been officially explained by the British police. There have been no charges in this case so far. The counter-terror police have gone so far only to say that this case is hugely complex … [and that] multiple lines of inquiry are being pursued.”

Since March, the US has used the RAF Fairford airbase to launch attacks on Iran, and the UK has authorised US bombers based there to strike Iranian missile sites targeting ships in the Strait of Hormuz.

Iran’s Islamic Revolutionary Guard Corps (IRGC), a powerful military force that operates alongside the country’s regular armed forces, warned in July that bases used to launch attacks on Iran could be treated as legitimate targets.