The Biden administration has offered to free convicted Russian arms merchant Viktor Bout and send him back to Moscow in exchange for detained Americans Brittney Griner and Paul Whelan, CNN reported Wednesday. US Secretary of State Anthony Blinken said the US had made a “substantial proposal,” but declined to provide more details about the possible deal.

In a news conference following remarks Wednesday, Blinken said that President Joe Biden “is prepared to take tough decisions if it means the safe returns of Americans.” Blinken said that the US presented its “deal on the table” to Russian officials in recent weeks but had yet “to get to yes.”

Blinken said he planned to contact Russian Foreign Minister Sergei Lavrov to press the deal. Blinken noted that he has not been in touch with Lavrov since the early days of Russia’s invasion of Ukraine — direct communications that the US purposely severed in protest of what they characterized as Moscow’s official duplicity and intransigence in the wake of Russian President Vladimir Putin’s move to invade Ukraine in February.

As Blinken spoke, CNN reported that “the plan to trade Bout for Whelan and Griner received the backing of President Joe Biden after being under discussion since earlier this year. Biden’s support for the swap overrides opposition from the Department of Justice, which is generally against prisoner trades.”

Relatives of both Whelan, who has been held by Russia for alleged espionage since 2018, and WNBA basketball star Griner, now on trial for drug possession, have pleaded with the White House to press their release. Griner pleaded guilty in a Moscow court in early July but said she unwittingly brought medicinal cannabis into Russia in her haste to appear their for a basketball exhibition. She could face up to 10 years in a Russian prison.

CNN quoted an unnamed senior administration official as saying that the US side “communicated a substantial offer that we believe could be successful based on a history of conversations with the Russians. We communicated that a number of weeks ago, in June.” The official added that despite the lack of response from Moscow, “we continue to communicate the offer at very senior levels.”

CNN also reported that the Justice Department initially opposed trading Bout, but “eventually accepted that a Bout trade has the support of top officials at the State Department and White House, including Biden himself.” Officials at the Drug Enforcement Agency, which led the international sting operation that ended with Bout’s arrest in Bangkok in 2008, are also known to oppose the proposal.

Bout was convicted in a federal court in New York on Nov. 2, 2011 on federal charges of conspiracy to aid a terrorist organization, deliver anti-aircraft missiles and kill American citizens and U.S. officials.

He was sentenced to a 25-year prison term on April 5, 2012 and would be due for release in 2029, according to his Bureau of Prisons locator, which as of Wednesday reported that he was still being held at the federal medium-security penitentiary at Marion, Ill., where he has been jailed since 2012.

Bout, a former Soviet military officer, amassed a fleet of more than 60 transport planes, hundreds of companies and a fortune reportedly in excess of $6 billion. His aircraft flew from Afghanistan to South America, carrying numerous cargoes that ranged from gladiolas and frozen fish to drilling equipment and illicit diamonds.

But the Bout air empire’s stock in trade, according to authorities, was often black market arms and sophisticated weapons systems that his planes provided to warlords and dictators who included Libya’s Moammar Gadhafi, Liberia’s Charles Taylor, Zaire’s Mobutu Sesse Seko and Taliban founder Mullah Omar. Bout’s planes even flew under contract for the Bush administration after the US invasion of Iraq in 2002 despite Treasury sanctions that targeted Bout, his companies and allies in his orbit.

That ended in 2008, when a year-long DEA sting operation led to Bout’s arrest in Bangkok while he was negotiating with US undercover informants posing as South American narco-terrorists. His arrest that March has kept Bout jailed for the past 14 years.