An Iranian-American citizen has been granted a brief leave from Tehran’s notorious Evin prison after being hospitalized, his American lawyer said January 28.

Jared Genser said in a statement that his 81-year-old client, Baquer Namazi, would be released from Evin on a four-day furlough.

Genser, who also represents Namazi’s imprisoned son Siamak, has admitted that he does not know why the Islamic Republic suddenly decided to allow the elderly dual national to leave the prison.

"We do not know why the Iranian government has released Baquer Namazi for only four days. But what is clear is that returning him to Evin Prison would be a death sentence that would be carried out quickly," Genser said, adding, “His release must be made permanent on humanitarian grounds, and it is urgent that Siamak Namazi and the rest of the wrongfully imprisoned Americans also have their cases resolved."

Meanwhile, Undersecretary of State Steve Goldstein described the temporary leave as a “first step,” but insisted the 81-year-old must be allowed to return to the United States due to his poor health.

"We welcome the release of Baquer Namazi given his deteriorating health, but we note that his release is only temporary,” said State Department spokeswoman Heather Nauert in a statement. “We call for the immediate and full release of the Namazi family, including his son Siamak, as well as other Americans unjustly held by the Iranian government.”

Namazi was hospitalized earlier last week due to low blood pressure and irregular heartbeat. It was the fourth time he had been taken to the hospital in the past year. Namazi was arrested in February 2016 on espionage charges, allegations he denies.

Namazi,an ex-UNICEF official and former governor-general of the Khuzestan province in southwestern Iran before the Islamic Revolution, underwent emergency heart surgery to install a pacemaker in September 2017.

In October 2015, Islamic Revolutionary Guard Corps (IRGC) intelligence agents detained Namazi's son Siamak, a businessman in his mid-40s who also holds dual U.S.-Iranian citizenship, while he was visiting family in Tehran.

Hoping to help with his son’s release, Baquer also returned to Tehran, but was arrested soon after his arrival.

Father and son were each sentenced in the autumn of 2016 to 10 years in prison for spying and cooperating with the U.S. government. They have categorically denied the charges.

The Namazis are not the only Iranians with dual citizenship who are currently behind bars in Tehran.

Five other U.S. citizens and permanent U.S. residents have been arrested in Iran over the past two years: Princeton doctoral student Xiyue Wang, art gallery owner Karan Vafadari and his wife Afarin Niasari, Robin Reza Shahini, an Iranian-American from California, and Nizar Zakka, a Lebanese national with U.S. permanent residency.

Last July Trump warned that Iran would face "new and serious consequences" unless all unjustly detained American citizens were released and returned.

Baquer’s younger son Babak Namazi appealed to President Trump to intervene in his family's case. "My family desperately needs the president to redouble his efforts to bring Siamak and my dad home," Babak said.

Iranians with U.S. citizenship are not the only dual nationals who have been arrested during visits to the Islamic republic in recent years.

Nazanin Zaghari-Ratcliffe, a 39-year-old project manager with the Thomson Reuters Foundation, was arrested and separated from her 2-year-old daughter by the IRGC in April 2016 at Tehran's airport as she was about to return to the United Kingdom after a family visit.

​The mother was convicted of plotting to topple Iran's clerical system of government and sentenced to five years in prison in September 2016.

Both her family and the Thomson Reuters Foundation have denied the charges.

Kamal Foroughi is another British-Iranian currently behind bars in Evin Prison. Iranian authorities arrested him in May 2011 while he was working in Tehran as a consultant for the Malaysian national oil and gas company Petronas. In 2013, he was sentenced to eight years in prison—seven for espionage and an additional year for possessing alcohol in his home.

Ahmad Reza Djalali, a physician and an Iranian-Swedish citizen, is in Evin awaiting a death sentence for spying against the Islamic Republic and collaborating with the Mossad to assassinate Iran’s nuclear scientists, a charge that has been denied by Djalali and dismissed by Israel as totally unfounded.

On January 16, 2016 Tehran released four Iranian-American prisoners including Saeid Abedini, who was arrested for converting to Christianity, former U.S. marine Amir Mirzaei Hekmati, and The Washington Post’s Tehran Bureau Chief Jason Rezaian.

The Obama administration reportedly secretly airlifted $400 million in cash to Tehran at the same time as their release, although the administration later said the two events were unconnected.

The money represented the first installment of a $1.7 billion settlement that the Obama administration reached with The Islamic Republic to resolve a decades-old failed arms deal signed during Shah Mohammad Reza Pahlavi’s reign, long before the Islamic Revolution.

“With the nuclear deal done and prisoners released, the time was right to resolve this dispute as well,” President Obama said at the time, without revealing the $400 million payment.

Nevertheless, many Republicans, including Senator Tom Cotton, accused the Obama administration of paying ransom to Iranian clergy to free the Americans.

“This break with long-standing U.S. policy puts a price on the heads of Americans and has led Iran to continue its illegal seizures of US citizens,” Sen. Cotton told the Wall Street Journal.

Iran does not recognize dual citizenship and judiciary officials have said all dual nationals behind bars in Iran are treated as Iranian citizens in the legal process.