ASSOCIATED PRESS Jason Rezaian, The Washington Post's Tehran Bureau Chief, landed in Geneva on Sunday after being imprisoned in Iran for over a year. His brother Ali, pictured, had been a vocal advocate for his release.

WASHINGTON -- In early December, six House Democrats traveled to New York for an under-the-radar meeting with Iran's U.N. Ambassador Gholamali Khoshroo. The topic: Four Americans being detained by Iran.

Notified the night before that Khoshroo would make time to meet with the members during their United Nations visit, Rep. Jared Huffman, a Democrat of California, immediately hopped on a plane. When Huffman arrived, his House colleagues handed the platform over to him.

Sitting in the room with Khoshroo -- who joined the meeting unaccompanied by any staff -- Huffman, and other members of the Congressional Progressive Caucus, got straight to the point: Iran needed to release Washington Post journalist Jason Rezaian, former U.S. Marine Amir Hekmati, Christian pastor Saeed Abedini and retired FBI agent Robert Levinson. The latter has yet to be released and his whereabouts remain unknown.

Huffman represents California’s 2nd congressional district, which includes Marin County -- where Rezaian and his brother Ali, who has led the push for Rezaian’s release, were born and raised.

“I flew to New York specifically for that part of the [day’s] meetings,” Huffman told The Huffington Post on Sunday -- one day after the U.S. secured the release of Rezaian and three other Americans, including Nosratollah Khosrawi-Roodsari, whose imprisonment was not reported previously.

“My points were very direct,” Huffman continued. “I basically explained that even the most progressive members of Congress were not going to let go of this issue of the wrongly imprisoned Americans, and it was going to be impossible to improve relations as long as they arbitrarily rounded up and imprisoned journalists.”

The tone of the roughly 30-minute meeting, which included Reps. Mark Pocan (D-Wis.), Jan Schakowsky (D-Ill.), Steve Cohen (D-Tenn.), Suzanne Bonamici (D-Ore.), and Bennie Thompson (D-Miss.), was “positive,” according to a source in the room, who spoke on condition of anonymity due to the sensitivity of the meeting. Huffman described Khoshroo as a “quiet” and “not particularly warm or engaging” man, but he left the conversation with hope.

ASSOCIATED PRESS Rep. Jared Huffman (D-Calif.) represents Jason Rezaian's hometown, and told Khoshroo during the December meeting to tell Iran's "supreme leader" to release Rezaian and the other Americans soon.

One week prior, Huffman and Rep. Dan Kildee (D-Mich.) met with Giulio Haas, the Swiss ambassador to Iran. From those two back-to-back meetings, Huffman saw a trend emerge. Both officials were hinting at a “choreography” between the impending implementation day on which the U.S. -- in line with the nuclear deal -- would lift sanctions related to Iran’s nuclear program, and when the prisoners would finally be released.

“[Khoshroo] reaffirmed, to some extent for me, that while some people thought the Iranian election was the key factor in the release of the prisoners, and that we would have to wait until after that election, implementation day was a much more significant event,” Huffman said. Iran’s election isn’t until the end of February.

Setting up the meeting with Khoshroo wasn’t easy; it took months, Huffman said, but gave him the chance to ask the ambassador to take his message “directly to the Supreme Leader” -- Ayatollah Ali Khamenei, who is the final say on major decisions.

According to Huffman, Khoshroo said he and Iranian “President Hassan Rouhani thought very highly of Jason and wanted to see Jason released.” The issue, Huffman said, was that there appeared to be some “internal politics at play,” specifically “division within the Iranian government, and the fact that hardliners in the Revolutionary Guard were driving the contingent imprisonment of all these innocent Americans.”

Huffman said in his conversation late last year with Swiss ambassador Haaus, he came to realize how “advanced” the discussions over the prisoner exchange were, and that “significant proposals and counterproposals had been made.”

As it became steadily clear everyone was moving forward, though, Huffman said, negotiators knew Rezaian would become less "politically valuable" to the hardliners.

While the meeting focused on the Americans imprisoned by Iran, other concerns were raised by the members in attendance. Khoshroo and the members discussed the interest in Syria's civil war by their two countries. Lawmakers also drew a hard line on Israel, expressing their strong support.

The six members told Khoshroo that Iran's backing of Hamas and Hezbollah, and "recurring anti-Semitic rhetoric" from its leaders, were "major impediments" to forging a better relationship. That "behavior" the members said "needed to change," according to the source in the room.

JEWEL SAMAD via Getty Images Iran's envoy to the United Nations Gholamali Khoshroo told the progressive Democrats that Iran believes a majority of Congress opposes diplomacy.

The source said Khoshroo made one brief comment that “negative rhetoric from folks on the right made diplomacy more difficult for [Iran]," which believes a majority of Congress is filled with hardliners who oppose diplomacy. Indeed, the Republican Party, which controls the House and the Senate, unanimously opposed last year’s nuclear agreement with Iran. That’s why the progressive members wanted “direct parliamentary dialogue” of their own to send a “strong” message that they supported diplomacy efforts and improving relations with Iran from the beginning.

The nature of the meeting, the source added, was "fundamentally different" than those between the State Department and Iranian negotiators because it was more about advocacy, and offered the perspective of members.

Now, six weeks after that meeting, the freed Rezaian, Abedini and Hekmati arrived in Geneva on Sunday. The fourth freed American citizen, Khosrawi-Roodsari, was not on the flight.

Describing the emotions he felt after getting news from the White House the day prior about the release, Huffman said a wave of “elation, relief -- great relief” fell over him.

“There were a lot of false starts and false hopes, but something about the timing, again in light of the conversations, felt real this time,” he said. “Everything is not fixed, but certainly events like this and the successful return of our 10 sailors build the kind of normalcy and confidence that can help improve relations.”

Jessica Schulberg contributed reporting.

CORRECTION: A previous version of this story incorrectly identified Rep. Mike Thompson (D-Calif.) as one of the lawmakers who met with Khoshroo.