WASHINGTON (AP) — Bill Richardson, the former U.S. ambassador to the United Nations and a frequent emissary in hostage negotiations, said Tuesday he was hopeful about the chances of a two-for-two prisoner swap that could result in Russia’s release of WNBA star Brittney Griner and another jailed American, Paul Whelan.

In cases like this, Richardson said in an interview with The Associated Press, “it’s proportional — two-for two.”

Richardson, who traveled to Russia in advance of the release by Moscow of Marine veteran Trevor Reed in an April prisoner swap, declined to discuss the current status of negotiations with Russia over Griner or Whelan or to explain what role he may be playing in the talks.

But he said he felt “relatively positive” for several reasons. For one, the conclusion of the criminal case against Griner last week — she was convicted and sentenced to nine years in prison on drug charges — provides an opening for diplomatic negotiations to begin in earnest. Plus, he said, the U.S. and Russia have already shown a willingness to carry out prisoner swaps, as evidenced by the Reed exchange. And Griner’s strategy of expressing contrition before a Russian court was important.

Secretary of State Antony Blinken took the unusual step of revealing publicly last month that the U.S. in June made a “substantial proposal” to get Griner and Whelan home.