Belgium, France and Germany have detained six people over an alleged plot to bomb a weekend rally by exiled Iranian opposition groups in Paris. What do we know about this highly sensitive, murky case?
Who has been arrested?
Federal prosecutors in Brussels first revealed the arrests on Monday afternoon after charging a husband and wife who were described as Belgian nationals of Iranian origin.
Aged 38 and 33 and identified by their first names Amir and Nasimeh, they were said to have been planning a bomb attack on Saturday at a conference organised by the National Council of Resistance of Iran (NCRI) in a Paris suburb.
Belgian police found 500 grams of TATP, a highly unstable but powerful explosive, as well as a detonator inside their Mercedes car when they were stopped in a leafy residential area of the Belgian capital on Saturday.
Separately, German police swooped on an Iranian diplomat based in Vienna who has been named as Assadollah Assadi by Iranian opposition groups, who also claim he has been an intelligence agent in Austria since 2014.
The Austrian government called Tuesday for Iranian authorities to lift his diplomatic immunity.
French police arrested another three people on Saturday, although two of them were released on Monday, French legal sources said.
What was the alleged target?
Thousands of Iranian opposition supporters from all over Europe, many of them bussed in for the event, gathered Saturday at the Paris Convention Centre just north of the French capital for the NCRI's annual meeting.
The NCRI is an umbrella group for exiled opposition organisations, including the former rebel People's Mujahedin, which was once considered a terror group by European and American authorities and is banned in Iran.
The People's Mujahedin was formed in the 1960s to overthrow the Shah of Iran and it continues to organise opposition to the current leaders of the Islamic republic who took power following the 1979 revolution.
Special guests at this year's conference included former New York mayor and Trump lawyer Rudy Giuliani and US Republican firebrand and former House speaker Newt Gingrich, who both urged regime change in Iran.
The NCRI immediately pointed the finger at Tehran, saying the regime was behind the alleged plot.
Has Iran been blamed for attacks before?
The US considers the country to be world's biggest state sponsor of terrorism because of its links with a host of armed groups, particularly the powerful Hezbollah militia in Lebanon and Hamas in the Palestinian territories, as well as other networks in Iraq and Yemen.
Hezbollah has been blamed for a string of attacks on Western targets, particularly American and Jewish ones, and US intelligence services consider that the group acts in step with Iranian intelligence services.
Argentinian prosecutors pointed the finger at Iranian officials and Hezbollah over the 1992 bombing of the Israeli embassy in Buenos Aires, in which 29 people were killed. Two years later, an attack on a Jewish community centre in the city left 85 dead.
More recently, in 2012, Iranians were identified by Indian authorities as being behind an assassination plot on an Israeli diplomat that saw a hitman attach a magnetic bomb to an embassy car in New Delhi.
That incident came shortly after two Iranian men were arrested in Thailand over a suspected plot to attack Israeli diplomats in Bangkok.
So is Tehran likely to be behind the plot?
French diplomats and security analysts remain extremely cautious and several of them refused to talk on the record when contacted by AFP because of the extreme sensitivity of the case and the lack of clarity.
The timing of the alleged plot is noteworthy it came just as Iranian President Hassan Rouhani was arriving in Europe, hoping to secure continued support for the 2015 nuclear agreement.
The European Union has been a strong backer of the deal, which offers Iran sanctions relief in return for accepting curbs on its nuclear programme, despite US President Donald Trump's decision to pull out in May.
Evidence that Iran had sought to carry out a bombing on European soil would seriously jeopardise those efforts and make it politically more difficult for leaders such as France's Emmanuel Macron to continue to support the accord.
Iran's foreign minister implied that the regime's enemies -- notably Israel or the United States -- might be behind a "false flag ploy", meaning a deliberate effort to discredit Iran.
"How convenient: Just as we embark on a presidential visit to Europe, an alleged Iranian operation and its 'plotters' arrested," Mohammad Javad Zarif tweeted.
"Iran unequivocally condemns all violence and terror anywhere, and is ready to work with all concerned to uncover what is a sinister false flag ploy," he said.
Investigations in France, Germany and Belgium will perhaps shed light on the real culprits, although the facts may never emerge from the murky world of intelligence operatives and covert operations.
Thousands of Iranian opposition supporters from all over Europe, many of them bussed in, attended the annual meeting of the CNRI