Baquer Namazi, 80, is the father of American businessman Siamak Namazi, who was detained in October and was not part of last month's exchange. Before his father's arrest, he was the last confirmed U.S. prisoner still being held in Iran.
Baquer Namazi, a former UNICEF official
, was arrested on Monday, his wife Effie Namazi said on Facebook.
He is an Iranian-American.
"I must share the shocking and sad news that Baquer was arrested in Tehran late evening of 22 February 2016 and as far as I have been told by those who took him taken to Evin prison," she wrote. "Now both my innocent son Siamak and my Baquer are in prison for no reason. This is a nightmare I can't describe."
CNN has reached out to Iranian authorities, who have not confirmed Baquer Namazi's arrest.
Effie Namazi complained in her Facebook post that she has been unable to get further details about her husband and that a lawyer for the family has not been allowed to see him.
"I am extremely concerned and worried sick for Baquer's health since he is an 80 year old man and has a serious heart and other conditions which requires him to take special heart and other medicine," Effie Namazi wrote.
After last month's prisoner swap, Iranian-American groups wrote to Secretary of State John Kerry, urging the United States to do more to secure Siamak Namazi's release.
But America's intelligence chief James Clapper suspected Iran has an interest in holding on to Americans.
Iran "might attempt to use any additional U.S. citizens" held in Iran "as bargaining chips for U.S. concessions," Clapper, the director of national intelligence, wrote in a worldwide threat summary submitted to Congress on Tuesday.
Iran's Supreme Leader Ayatollah Ali Khamenei sparked a crackdown on journalists and businessmen with suspected ties to the West after publicly claiming the United States was using the recent nuclear deal reached with Iran
to "infiltrate and penetrate" Iran, according to Clapper's testimony.
"The crackdown was intended by hardliners to demonstrate to (Iranian) President (Hassan) Rouhani and to Washington that a broader opening to the West ... would not be tolerated."
Namazi not freed
Siamak Namazi was arrested the same month the nuclear agreement was reached and was incarcerated in Evin prison, which became notorious for torture allegations during the street marches of Iran's Green Movement in 2009.
Among the U.S. citizens released last month
in the wake of the nuclear deal were Washington Post reporter Jason Rezaian, Christian pastor Saeed Abedini and Marine veteran Amir Hekmati. Namazi remained behind bars.
Another man, who was previously unknown to U.S. authorities, was also released. He did not go to the United States but stayed in Iran.
A former FBI agent, Robert Levinson
, went missing in Iran in 2007, but his whereabouts have been a mystery, and Iran has denied holding him.