United Kingdom police have arrested a 25-year-old British-Iranian man in relation to a security incident at a Royal Air Force (RAF) station near Fairford, England, this week

The man, described by police as a “dual UK/Iranian national” and detained on suspicion of preparing “terrorist” acts, is the sixth person to be arrested in what British authorities said was potentially an Iran-backed plot. The five other people arrested earlier were released on bail on Monday.

The investigation began when British authorities were alerted early on Sunday about unusual activity near the airbase, including oddly parked vans and a group of masked men. After searching the vans, authorities found no weapons or explosives but did recover a “quantity of petrol”.

The RAF station has been used by the United States for military operations, including by bombers during the US-Israel war on Iran.

Here’s what we know about the latest development in the case:

What’s happened?

British counterterrorism police announced the arrest of a 25-year-old British-Iranian dual national in Westminster, London, on Thursday. Iran, however, does not recognise dual citizenship, and treats Iranians with a second passport solely as Iranian.

The man was detained on “suspicion of preparation of terrorist acts under Section 5 of the Terrorism Act 2006”, the same offence cited in the arrests of the five other suspects. In line with standard police practice, none of the suspects has been named ahead of any charges being made.

Police said they also conducted searches of two properties in the London area. They appealed to the public for any information that could assist the investigation.

What do we know about the suspect in custody?

The Times newspaper reported that the British-Iranian suspect is being investigated as a possible “recruiter” in the still-opaque plot or as someone otherwise believed to have played a significant role.

The media reported that the man was placed under surveillance after the arrest of the first five men. He has not been named.

What evidence is there that this was an Iran-linked plot?

The UK has pointed to an Iranian connection but has so far kept the reason for that assessment under wraps.

On Wednesday, Prime Minister Andy Burnham said there are “strong indications” of Iran’s involvement in the incident and an “ongoing, complex and serious police investigation” is continuing.

He did not give information on what the suspects may have been planning or what intelligence has led the UK to suspect the involvement of Iran, saying only that he would “update people more in due course”.

In the US, President Donald Trump said the case “might be” Iran-linked and the US had been keeping tabs on the suspects “for a long time”. Trump criticised the UK’s decision to release the first five suspects on bail.

In Israel, Prime Minister Benjamin Netanyahu said his government had forewarned the UK about a potential “Iranian-sponsored attack” but did not specify if that was related to the same RAF base incident.

Iranian officials have firmly denied any connection to the incident. Deputy Foreign Minister for Political Affairs Majid Takht-Ravanch called the claims “false and ridiculous”.

Iran’s Ministry of Foreign Affairs summoned British Ambassador Hugo Shorter on Thursday to protest against the allegations.

What happened at RAF Fairford?

Authorities received two early-morning calls on Sunday alerting them to suspicious activity near the Fairford airbase, including three vans parked strangely and a group of hooded, masked men.

Unusually, the first call to the UK’s emergency line was made by one of the suspects who would later be arrested.

Police responded and arrested five British men in their early 20s in the nearby Whelford area, saying they were suspected of explosives offences. Counterterrorism police later took over the investigation and said the men were suspected of preparing “a terrorist act”.

The suspects were released on bail on Monday but remained subject to “stringent conditions on their movement and contact with others”, according to the UK’s head of counterterrorism policing, Laurence Taylor.

After searching the vans located near the airbase, police said they found no explosives but did retrieve a “quantity of petrol”.

Why were the men arrested at the scene given bail if this is a major incident?

That’s one of the big unanswered questions.

Under British “counterterrorism” law, police may hold suspects for up to two weeks before charging them. However, if authorities find little evidence to support their initial suspicion of “terrorism”, they have far less time – usually 24 to 36 hours, Chris Phillips, a former head of the UK’s National Counter Terrorism Security Office, told Al Jazeera.

Other security experts told Al Jazeera that police likely released the first five suspects because they did not deem them to be an imminent security risk or found that their potential involvement in any plot was unwitting.

“In cases involving terrorism, suspects can still be released on bail when the authorities feel relatively confident that these individuals do not pose an imminent danger of either carrying out or organising a terrorist attack,” Andrew Silke, professor of criminology at Royal Holloway, University of London, told Al Jazeera.

“Individuals may have been involved unknowingly or in a peripheral manner to potential terrorist activity. This seems possible in the current case,” he added.

Afzal Ashraf, a visiting fellow in international relations and security at Loughborough University and a former senior RAF officer, agreed that police likely have assessed that the five released suspects do not pose “any immediate danger”.

“We can reasonably assume that police judged that these individuals did not present any immediate danger nor did they have any credible means or intention of creating further danger to the public or to government installations,” Ashraf told Al Jazeera.

However, he said their release on bail should not necessarily be taken to mean the incident in question “was minor or that the Iranian-involvement hypothesis has been rejected”.

What happens next?

British authorities said they are continuing to investigate all aspects of the case, including the possibility of it being a foreign-backed “plot”.

Silke said authorities will likely be thoroughly analysing all suspects’ communications and interactions, looking for information on contacts who may have organised their involvement and for any payment the suspects may have received.

“The authorities can investigate all of these links to establish if they lead back to foreign government sources or agencies,” he said.

Silke added that authorities will also look into any links between the RAF airbase incident and previous plots carried out on British soil.

Phillips said he believes the suspects could have been recruited online by foreign actors to carry out some type of malicious activity, such as “setting fire to something”.

He said both Russia and Iran have allegedly paid “stooges” to carry out such acts in recent years. “At first glance now, it seems that that may be the case, but time will tell,” Phillips added.

Iran and Russia have also previously been accused of orchestrating plots in the UK. Those plots have involved “surveillance, sabotage, arson or physical violence”, MI5 Director General Ken McCallum said last year.