Hours after reports surfaced that the U.S. government discreetly airlifted $400 million to Iran in January just as long-held American hostages in the Islamic Republic were being released – a payment some have called a “ransom” – the son of a man still thought to be held in Iran is asking why his dad wasn’t part of the deal.

Tuesday’s Wall Street Journal report of the previously undisclosed money transfer was termed “paying ransom to kidnappers” by Sen. Mark Kirk, R.-Ill., one of several lawmakers who expressed shock and dismay at the news. State Department spokesman John Kirby, however, said the cash was not related to the release of four Americans – Washington Post reporter Jason Rezaian; ex-Marine Amir Hekmati; Christian pastor Saeed Abedini and Nosratollah Khosravi-Roodsari. Instead, Kirby said the nearly half a billion dollar airlift was the first installment in a $1.7 billion settlement to resolve a dispute that dated back to 1979.

But Dan Levinson isn’t questioning the purpose of the money; he just wants to know why his father hasn’t come home.

“As part of this [Iran nuclear deal], Iran promised to do everything they could to bring my dad home and so far they haven’t followed up,” Levinson told Greta Van Susteren during "On the Record" Wednesday night. “I went over there, they didn’t have any answers.”

Levinson’s father, Bob Levinson, an ex-FBI agent and CIA contractor, disappeared on an Iranian island in March 2007. Iran has never admitted to holding Bob Levinson captive, which his son thinks could be the major hurdle in bringing his father back.

“They’re not saying that and that has been a roadblock for everything,” the younger Levinson said.

White House Press Secretary Josh Earnest said in January that Bob Levinson may no longer be held in Iran. But Dan is confident the Iranians have custody and maintains that his father is alive.

“We believe he’s alive, the FBI believes he’s alive, they’re tracking every lead and they said there’s no reason to believe he’s not alive,” said Dan Levinson, who last received a “proof of life” photo of his father in April 2011. “And I think the [Obama] administration believes he’s alive.”

Sen. Tom Cotton, R-Ark., told “On The Record” that the failure to bring Bob back lies squarely at the feet of the Obama administration.

“My heart breaks for the Levinson family and, as I’ve long said, we should have negotiated for the release of all the hostages, including Mr. Levinson, who is the longest-held hostage in Iran,” Cotton said.

Dan said his family has never lost hope, but expressed frustration with the nearly decade-long saga.

“Every day it’s a struggle,” Dan said. “And every day we’re trying to figure out how to get him home.”