Over the past few weeks, Greta Van Susteren has highlighted the plight of a former U.S. Marine who has been held in an Iranian prison for more than three years.

Sgt. Amir Hekmati, an Iranian-American with dual citizenship, was arrested while on a trip to Iran to see his ailing grandmother and accused of being an American spy.

He was convicted on a lesser charge and sentenced to 10 years in prison. His family says he has been tortured by Iranian authorities.

In the special hour, Greta will talk to Amir's brother-in-law, who called it "hurtful" that more is not being done to free the former Marine. 

"We think Amir is worth fighting for, aside from being a Marine," said Dr. Rami Kurdi.

"He’s an American citizen and top of that he’s a man who served his country, who did his time, put his life on the line. He would do it today. He’s that kind of person and it’s obvious. He’s a decorated Marine and to know that more was not being done, that we had to act through the Swiss, it was hurtful to be honest."

Also on the show, Greta talks to Montel Williams, who has spoken out forcefully in support of Amir, and Democrat Rep. Dan Kildee, who has been pushing the administration to do more to free Amir. 

And you'll see more from Greta's trip to Michigan, where she sat down for an exclusive interview with Amir's parents and family members.

Tune in to "Marine Held in Iran," tonight at 7p ET and spread the word on social media using #FreeAmirNow.

Watch a sneak preview above and check out more of the coverage from "On the Record" below. 

Montel: How Dare We Make a Deal With Iran While Americans Are Still Held?!

'Where Is Everyone?' Greta Urges Media to Report on U.S. Vet Jailed in Iran

'Amir Is Innocent': Jailed Marine's Mom Makes Tearful Plea to Iran