Siamak Namazi, an Iranian American detained in Iran, has been sent back to prison after the Islamic Revolutionary Guard Corps did not extend his temporary release.
"Yesterday, members of the IRGC informed Siamak Namazi that his furlough would not be extended further, without providing any explanation," said the Namazi family's lawyer, Jared Genser.
Mr Namazi, 51, is the longest-serving of the four Iranian Americans held in Tehran.
He was detained in October 2015 under a conviction for espionage. His father, Baquer Namazi, a former Unicef official, was detained the following year, also on charges of espionage. Both deny the allegations.
Mr Namazi, who has been held in Iran for nearly seven years, was granted release from prison after his father, who was serving his sentence under house arrest, was allowed to travel to Abu Dhabi for medical treatment.
His release came as indirect talks between Iran and the US over how to revive the 2015 nuclear deal have stalled and as Tehran’s leadership faces mounting pressure at home while anti-government protests continue to grip the country amid a violent crackdown by security forces.
Siamak Namazi's return to Evin prison came as a blow to his family. After years of hoping, they had finally felt progress in a difficult case that the US State Department has been unable to resolve.
“I was genuinely hopeful for the first time that my father’s departure was the beginning of a new, less painful chapter in the struggle to make our family whole again," Siamak Namazi’s brother Babak said.
"But Siamak’s return to Evin has shattered that hope."
The Namazi family's long ordeal with the Iranian justice system started in 2015, when Siamak Namazi, a businessman, was arrested during a trip.
His father was arrested when he tried to visit his son in prison in 2016.
The two were sentenced to 10 years in prison on what the US State Department has called "baseless charges"
Mr Namazi had already had his furlough extended once, giving the family some hope that it would continue.
Mr Genser, who represents the family for free, condemned Iran's decision to return Mr Namazi to prison.
"Iran’s decision to refuse to renew Siamak’s furlough is devastating but, ultimately, unsurprising," he said.
"For Iran to use Baquer’s departure and Siamak’s temporary release to portray itself as acting in good faith, only to immediately and needlessly throw him back behind bars, is a telling display of the precarious situation of the hostages."