EXCLUSIVE: American Prisoner in Iran Taunted After Obama's White House Correspondents' Dinner Speech

— -- Just hours after President Obama used his appearance at the White House Correspondents' Dinner to call for the release of an American journalist held prisoner in Iran, another American held at the same prison was taunted by Iranian prison guards who told him the president did not mention his name, his family said.

The prisoner, Marine Corps veteran Amir Hekmati, called his mother over the weekend from the notorious Evin prison in Tehran, terrified that gaining his release is not a priority for the U.S. government, his family said. Now, in an emotional letter to the White House, Amir’s sister is demanding to know why the president has never said her brother’s name in public. He has been imprisoned for nearly four years.

“He has already been mistreated, abused, and tortured,” writes Sarah Hekmati, Amir’s sister, in a letter to White House counter-terrorism advisor Lisa Monaco. “Now the mental torture continues as he is made to feel that the country he put his life on the line for, the one he defended, and the president he voted for has left him behind and are not actively trying to secure his freedom.”

Of the three Americans known to be imprisoned in Iran, Hekmati has been held the longest. He was arrested in 2011 when, according to his family, he was visiting his ailing grandmother in Iran. He was sentenced to death in January 2012 for “espionage, waging war against god and corrupting the earth.”

President Obama spoke out for the release of American journalist Jason Rezaian at the White House Correspondents’ Dinner and earlier this year he spoke out for the release of Pastor Saeed Abedini at the National Prayer Breakfast. The Hekmati family said they have repeatedly asked the White House to push for Amir Hekmati’s release.

“Why has President Obama yet to utter the name Amir Hekmati?” his sister wrote. “Why on days significant for Amir -- Memorial Day, Veteran's Day, the anniversary of his death sentence, the anniversary of his imprisonment -- President Obama cannot say the name Amir Hekmati out loud, but he can say it for Jason Rezaian and he can say it for Pastor Abedini? Why when we make a request is it ignored? Why am I forced to write this email to you AGAIN, the same subject AGAIN, the same plea AGAIN?”

ABC News posed her questions to the White House today and was told by Press Secretary Josh Earnest that “each case and the efforts that we're undertaking to secure their release is treated independently.”

“Certainly when considering how best to secure the release of these individuals, a calculation is made about the wisdom of the publicity that surrounds the efforts to secure their release,” Earnest said.

The president did mention Hekmati in a March written statement about the U.S. citizens detained or missing in Iran and has personally raised Hekmati’s case during a phone call with Iranian President Hassan Rouhani in September 2013, but he has never mentioned his name or his case in any of his public remarks.

“The concern for the well-being of those individuals is shared by everybody here at the White House. We've made clear what those concerns are to the Iranian government. And we're going to continue the effort to try to secure the release and safe return of these individuals,” Earnest said.

Meanwhile, the Hekmati family waits for answers.

“Please spare us this dignity and give us a straightforward answer as to why in nearly 4 years President Obama has [not] raised Amir’s plight individually outside of the context of the others imprisoned. Not even once. Not even when he was sentenced to death. The only question at this point is why,” Sarah Hekmati wrote.

The Hekmati family plans to bring attention to Amir’s plight on Capitol Hill later this week with television personality Montel Williams, who has championed their cause. Rep. Dan Kildee, D-Mich., is also working with the family and has arranged meetings with fellow lawmakers about the case.