A possible four-person prisoner exchange that would have included convicted arms merchant Viktor Bout was discussed between US and Russian diplomats last year but then derailed after Moscow insisted on adding a third Russian now in American custody, according to a new account.

Soon after US President Joe Biden and Russian President Vladimir Putin met for an arranged summit in Geneva last June, Bloomberg reports, American and Russian diplomats began private discussions about trading Bout and an unnamed Russian prisoner to Moscow for imprisoned Americans Paul Whelan and Trevor Reed.

Russian officials have openly pressed for the return of Bout and Konstantin Yaroshenko, another Russian imprisoned on narcotics charges, but Biden made no specific mention of the two Russians after the summit.

According to the Bloomberg account, “the two sides were negotiating to swap two former U.S. Marines imprisoned in Russia, Paul Whelan and Trevor Reed, for two Russians held in the U.S., including notorious arms dealer Viktor Bout.”

But the talks fell through, Bloomberg reports, after the Russians demanded that the deal be expanded to include a third Russian, Vladislav Klyushin, a Russian tech tycoon who was extradited to the US from Switzerland on Dec. 18. Klyushin is accused by the Justice Department of illegally making tens of millions of dollars trading on hacked corporate-earnings information.

Bloomberg reports that “Russian intelligence has concluded that Klyushin, 41, has access to documents relating to a Russian campaign to hack Democratic Party servers during the 2016 U.S. election.”

After Switzerland refused to hand Klyushin back to Russia, Bloomberg reports, “the Kremlin demanded that his name be added to the swap, according to three people with knowledge of the issue. That derailed the potential exchange” that included Bout. The proposed prisoner exchange “remains blocked,” Bloomberg reports.