Notorious for its conditions, Tehran's Evin prison is proving to be a headache for Western governments as a rising number of dual nationals are detained in what many see as a ruthless diplomatic strategy.
Fariba Adelkhah, a well-known academic with French and Iranian nationality, has been held in the prison in north Tehran since early June on charges that have not been disclosed.
Her arrest came just before an aide to French President Emmanuel Macron was visiting Tehran to salvage a multinational nuclear accord with Iran, which has been on life support since President Donald Trump withdrew the United States from it last year.
Some analysts see the arrests as part of the murky politics in Iran, with hardliners in the judiciary and the security apparatus scuttling the more conciliatory approach of moderates.
But others believe the government is setting a clear political goal with the arrests.
"It was ransom," Alireza Nader, founder of the New Iran foundation, said of Adelkhah's arrest.
When Iran "wants to increase leverage to open negotiations, especially with Western countries -- the US, the European countries like France -- it resorts to hostage-taking and uses them to negotiate, whatever objective they are trying to achieve," he said.
"It's been happening for the last 40 years," he said.
Several foreign citizens of Iranian heritage are currently behind bars in Iran, which does not recognize dual nationality.
At Evin, Adelkhah joined Nazanin Zaghari-Ratcliffe, a British-Iranian dual national who has been locked up since April 2016 and handed a five-year prison term for sedition.
An employee of the Thomson Reuters Foundation, the philanthropic arm of the global media group, the 40-year-old mother insists on her innocence.
Zaghari-Ratcliffe ended a 15-day hunger strike at the end of June and, adding to worries about her condition, her husband said she was transferred to the mental ward of a public hospital.
- Deals for prisoners -
The British press linked her arrest to Iranian efforts to seek $500 million owed by Britain over a tank sale that was canceled after the 1979 Islamic revolution toppled the pro-Western shah, although both London and Tehran denied a connection.
Iran has always denied that it uses dual nationals as means of pressure, although it has also publicly urged Western governments to take up the cases.
Iranian Foreign Minister Mohammad Javad Zarif, in an April appearance at a New York think tank, called for a prisoner swap for Iranians detained or threatened with extradition to the United States.
Jason Rezaian, who was the Tehran correspondent of The Washington Post, spent 544 days in Evin prison from 2014 to 2016 on accusations of espionage.
The Iranian-American reporter told AFP in February that he felt like a "pawn" in a geopolitical chess game as negotiations were underway on a nuclear accord.
"Each new farcical arrest is a reminder that taking hostages, 52 of them in fact, was the signature move of this regime when it first started 40 years ago," he wrote in his book "Prisoner," referring to the seizure of the US embassy after the revolution that led to the rupture of diplomatic relations.
He was freed on January 16, 2016 along with three other Americans as an international accord took force on Ian's nuclear program.
Then president Barack Obama granted clemency to seven Iranians and paid $1.7 billion to Iran for undelivered military equipment ordered by the shah.
The payment outraged Obama's Republican critics, who described it as a ransom and warned that it could encourage the imprisonment of more Americans.
The Obama administration argued that it was resolving a legal debt that would have become much more onerous for the United States without a resolution.