The family of Paul Whelan, an American held in Russia, welcomed the release of the basketball star Brittney Griner but lamented "a disappointment for us and a catastrophe for Paul".
"It's clear the US government has no concessions the Russian government will take for Paul Whelan," his brother David Whelan said.
News of Griner's release, in a swap for the arms dealer Viktor Bout, broke on Thursday morning.
At the White House, Joe Biden said: "She's safe, she's on a plane, she's on her way home after months of being unjustly detained in Russia, held under intolerable circumstances."
The president also said: "Sadly, for totally illegitimate reasons, Russia is treating Paul's case differently than Brittney's. And while we have not yet succeeded in securing Paul's release, we are not giving up. We will never give up."
Whelan, a corporate security executive from Michigan, has been held in Russia since 2018, on espionage charges the US and Whelan's family reject. In 2020, Whelan was sentenced to 16 years in jail. The US says he is wrongfully detained.
On Thursday, David Whelan said: "I am so glad that Brittney Griner is on her way home. As the family member of a Russian hostage, I can literally only imagine the joy she will have, being reunited with her loved ones, and in time for the holidays. There is no greater success than for a wrongful detainee to be freed and to go home."
Whelan said the Biden administration had made the "right decision" and added: "This time, US government officials let us know in advance that Paul would be left behind, unlike last April, when they left him."
Then, Trevor Reed, from Texas, was exchanged for a convicted drug trafficker.
David Whelan said: "That early warning meant that our family has been able to mentally prepare for what is now a public disappointment for us. And a catastrophe for Paul … Our parents have had calls with him every day since his return to IK-17 [a labour camp] on 2 December, and they will surely speak to him soon."
Paul Whelan's hopes had "soared with the knowledge that the US government was taking concrete steps for once towards his release", his brother said, to the extent that he had "been worrying about where he'd live when he got back to the US".
But, David Whelan said, he now could not imagine his brother "retains any hope that a government will negotiate his freedom at this point. It's clear the US government has no concessions that the Russian government will take for Paul Whelan. And so Paul will remain a prisoner until that changes.
"It will be the fourth Christmas Mum and Dad live through without Paul. They will be 85 and 83 on the fourth anniversary of his detention [28 December]. Time is Paul's, and our, enemy. The likelihood that our parents will see their son again diminishes each day his wrongful detention continues.
"Increasingly, I worry that Paul himself won't survive 12 more years in a Russian labour colony."
The statement linked to a fundraising page.
Whelan stressed that "we do not begrudge Ms Griner her freedom", adding: "Brittney's and Paul's cases were never really intertwined. It has always been a strong possibility that one might be freed without the other."
But, he said, "the US government needs to be more assertive. If bad actors like Russia are going to grab innocent Americans, the US needs a swifter, more direct response, and to be prepared in advance. In Russia's case, this may mean taking more law-breaking, Kremlin-connected Russians into custody.
"It's not like there aren't plenty around the world."