The family members of three Americans held captive in Iran testified before a congressional subcommittee on Thursday, asking President Trump and U.S. lawmakers to do more to free their loved ones.

Speaking to the House Middle East, North Africa and International Terrorism Subcommittee, Christine Levinson – she's the wife of retired FBI special agent Robert Levinson – made a personal appeal to the president to negotiate her husband’s release.

“My husband is the longest-held hostage in American history,” Levinson told lawmakers during the hearing. “I feel more could be done. It’s necessary for the Trump administration to make this a priority.”

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Agent Levinson, a specialist in Russian organized crime, was 58 when he boarded a flight in 2007 from Dubai and then made his way to Kish Island, Iran, a resort in the Persian Gulf. He's said to have been investigating cigarette smuggling -- and possibly working on a book -- although it has been reported that he was in Iran on a CIA contract for intelligence analysts at the agency.

After checking into the Maryam Hotel on Kish Island, Levinson met with an American fugitive, Dawud Salahuddin, also known as David Belfield and Hassan Abdulrahman. Soon after, Levinson disappeared and, despite denials, it is believed he is being held by Iranian intelligence officials.

“Iran has been able to feign ignorance time and time again with no response from the United States,” his wife said on Thursday.

Levinson’s wife asked why a number of U.S. hostages have been freed from captivity – including four Americans from Iran in 2016 – while her husband still remains in the Islamic Republic.

On Wednesday, President Trump met in the Oval Office with Danny Burch, an American oil worker who had been held in Yemen for almost two years.

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“We've gotten a lot of them out. … We have negotiations going on now -- I won't tell you where,” Trump said after the meeting. “I love doing it. This is the end result, a happy man with a happy family.”

Along with Levinson, the subcommittee heard testimony from Babak Namazi and Omar Zakka.

Namazi’s father, Baquer, and brother, Siamak, were sentenced in October 2016 to 10 years in prison for "collaborating with a hostile state" following a secretive trial that has since been condemned by the United Nations.

Zakka’s father, Nizar, is an information technology expert and legal permanent resident of the U.S. He was taken into custody by the Iranian Revolutionary Guard in 2015 after being invited to the country to participate in a conference on female entrepreneurship.

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“My father has been subjected to physical and mental torture,” Zakka told Congress. “We are tormented by the fear that something will happen to him or has happened to him.”

In response to the testimony from the families, the subcommittee chairman, Democratic Rep. Ted Deutch of Florida, introduced two resolutions: one calling for the immediate release of any U.S. citizens, legal residents and foreign nationals being held by Iran, and the second leveling sanctions against anyone in Iran responsible for the detention of people with ties to the U.S.

There are currently five Americans or U.S. legal residents being detained in Iran.