Diplomat who brought Brittney Griner home tells Paul Whelan: 'We haven't forgotten you'

U.S. Special Presidential Envoy for Hostage Affairs Roger Carstens, the official who brought Brittney Griner back to the U.S., told "Good Morning America" on Monday that he spoke to Paul Whelan just hours after landing in Texas with Griner.

"I landed at 4:30 in the morning, and at 9:30, Paul called from the penal colony in Russia. I explained to him, I said, 'Paul, it was one or nothing, we were not going to be able to get you out. We're going to keep working on it, but I understand you're a little frustrated with that," Carstens told "Good Morning America" co-anchor Robin Roberts.

"The president's focused. The secretary's focused. We're meeting today, Monday morning, to go through the next steps of the strategy, but Paul, we haven't forgotten you, we're coming to get you," Carstens added.

Whelan, a former Marine, has spent about four years in detention since he was seized in 2018 by Russia's domestic intelligence agency, the Federal Security Service, while visiting Moscow for a friend's wedding. He was sentenced to 16 years in prison on espionage charges but the U.S. and his family say that they were fabricated in order to take him as a political bargaining chip.

Biden's national security adviser, Jake Sullivan, said at Monday afternoon's White House press briefing that Elizabeth Whelan, Paul Whelan's sister, met with State Department officials over video that morning and called discussions with the Whelan family "substantive."

"They have had a number of very good questions and also a number of suggestions that they put forward, and we have been working to figure out what it is going to take to ultimately secure his freedom and how we can go about getting that and being able to sit down with the Russians and work out a deal," Sullivan said, adding the "specifics" of the discussions need to be kept in "sensitive channels."

Pressed on what exactly the administration can do to bring home Paul Whelan and if the U.S. would be willing to change policy to get him home, Sullivan said the administration would not take a different approach to Ukraine, if that was the implication.

"The big challenge we had over the course of the past several months is that what Russia was asking for to secure Paul Whelan's release was not something that we had to be able to give. That is a problem we are trying to solve. We have various ways that we are working through solutions, and we will be endeavoring on a daily basis from the president on down to finally develop a formula that works and that's as far as I can go today," Sullivan said.

Carstens, who was appointed to his position by former President Donald Trump in 2020, also said on "Good Morning America" on Monday that the work to bring Whelan home hasn't stopped and "was ongoing while we were on the plane coming home with Brittney."

Griner, a WNBA star who had pleaded guilty to drug charges after carrying a vape with hashish oil into Russia -- which she maintained was an inadvertent mistake -- was released last Thursday as part of a prisoner exchange between Moscow and Washington. In return for Griner's freedom from a penal colony, the U.S. released Viktor Bout, a notorious former arms dealer serving a 25-year sentence in Illinois following a conspiracy conviction -- prompting criticism from some.

Carstens countered on Monday to say U.S. national security experts did an assessment and determined that Bout being released was no longer a threat to U.S. security. He called decisions in a case like this "tough," but said "it's our moral obligation to get an American."

"I kind of flip it around, to me what's unacceptable is not Viktor Bout. What's unacceptable is an American wrongfully detained and held in a foreign jail cell. We have a moral obligation. Having a blue passport means something," he said. "While it's tough to make some of the decisions that we've made, we start from the premise and from the belief that it's our moral obligation to get an American."

Carstens also offered more details about the flight home with a talkative Griner following her release from a Russian penal colony last week. When he tried to show her to her seat on the plane, "she was having none of it," he said.

"She said, 'Look, I've been listening to Russian for ten months. I want to speak English, I want to chat with people,'" Carstens said. "And then as I tried to point her to her seat, she just walked past me and went right to the crew and started introducing herself to the crew members. And also, I watched her and she connected with everyone, looked in their eyes, shook hands, got to know their names, and only when that was all done she went back."

"I was really impressed at how she values the other people, to include the people that were trying to bring her home," he added.

Carstens said Griner spent about 12 hours of the 18-hour flight talking with others on the plane and slept for the other six.

"We talked about a lot of things, anything from her time held in the Russian penal colony, going through the trial process, obviously her love for Cherelle, her teammates, everything that she missed, the things that she misses about the United States and America," he said. "But I think I left with the impression that this is an intelligent, impressive woman who's very self-aware, very kind, very humble, and above all, authentic, and you get B.G. when you're talking to B.G."

ABC News' Molly Nagle contributed to this report.