This undated handout photo provided by the family of Robert Levinson shows the retired FBI agent who went missing on the Iranian island of Kish in March 2007. (Uncredited/via AP)

In testimony before a House committee Tuesday, the youngest son of a former FBI agent who disappeared in Iran a decade ago urged greater sanctions on Iran if it does not account for his father and release U.S. citizens imprisoned in the country.

Doug Levinson, who was 13 when Robert Levinson was last seen on Iran’s Kish Island, described being crestfallen that his father was not among five Americans freed as part of a prisoner swap to accompany the implementation of the Iran nuclear deal in 2016.

“Do not let Iran off the hook. They know exactly where he is,” Doug Levinson told a House Foreign Affairs subcommittee. “Hold their feet to the fire. Threaten them with sanctions. Do something. Do anything.”

Levinson was among three relatives of Americans imprisoned or missing in Iran who testified in support of a House bill calling for their freedom on humanitarian grounds. On Friday, the White House said President Trump is “prepared to impose new and serious consequences on the country if they are not released and returned.”

Iranian American businessman Siamak Namazi in San Francisco in 2006. (Handout/Reuters)

Iran is known to be holding at least four U.S. citizens and two permanent residents of the United States. U.S. officials have never been able to ascertain the whereabouts of Robert Levinson, whose family is convinced he is still alive, although Tehran insists it has no idea where he is or what happened to him.

Levinson entered Iran to gather information on government corruption on a mission for CIA personnel who did not have authority to run such overseas operations. Ten agency employees were disciplined because of the Levinson case. Iran has never acknowledged holding him.

The other Americans have been accused of espionage, charges that their families and the U.S. government say are baseless.

“My dad is innocent, and he will not be forced to do things against his will, including signing forced confessions,” said Omar Zakka, whose father, Nizar, is an Internet-freedom advocate arrested in 2015 when he traveled to Iran for a conference. He is in the fifth week of a hunger strike protesting his imprisonment.

“I fear for his life and safety,” his son said.

Iran has used Americans as bargaining chips ever since the 1979 revolution, when Iranian students stormed the U.S. Embassy and held 52 U.S. diplomats and citizens hostage for 444 days. Only this month, Princeton graduate student Xiyue Wang was sentenced to 10 years in prison after he accessed thousands of archived documents relating to his research on 19th- and early-20th-century Iran.

The prisoner swap that accompanied the implementation of the Iran nuclear deal in 2016, buying freedom for five Americans including Washington Post reporter Jason Rezaian, brought bitter disappointment for the families of those who were left behind.

Babak Namazi told the committee he was devastated that his brother, Siamak, was not among the Americans flown out of Tehran that night with the others. Their father, Baquer, a former UNICEF official, was arrested in early 2016 when he went to Tehran’s Evin prison hoping to visit Siamak. In October, father and son were sentenced to 10 years in prison for “cooperating” with a foreign government, meaning the United States.

Namazi said his father’s health is declining, and his brother, who has spent most of the past two years in solitary confinement, has grown despondent waiting for their release. He fears time is running out.

“The whole thing is crazy,” he told a small group of journalists Tuesday morning. “The whole thing is beyond comprehension.”

Last week, Iranian Foreign Minister Mohammad Javad Zarif told CBS News that the elder Namazi was not behind bars but also that he was not free to leave the country. But Babak Namazi said his father, who has been taken briefly out of prison twice to be hospitalized, remains in Evin.

Several Iranians are in U.S. prisons on sanctions-related charges. In recent weeks, officials in Tehran and Washington have suggested they may be willing to make a deal over the prisoners. Their fate has been brought up during bilateral meetings between U.S. and Iranian officials monitoring the nuclear deal and by allies from other countries who have met with Iranians.