Michael White, an American Navy veteran released from prison in Iran on medical furlough, is pictured in a March 25, 2020, photo released by his family. - (Courtesy White Family)By CONOR FINNEGAN, ABC News

(NEW YORK) -- An Iranian scientist detained by the U.S. was deported to Iran late Monday, but his release did not coincide with the freeing of any Americans detained by Iran -- dashing hopes for a prisoner swap after weeks of speculation and back channel talks.

At least four American citizens remain detained by Iran, including U.S. Navy veteran Michael White, who contracted the novel coronavirus in Iranian custody and was released on medical furlough. White was suspected to be the leading candidate for an exchange, even as tensions between the U.S. and Iran have remains intense and volatile.

Both the U.S. and Iranian foreign ministries denied on Tuesday that Sirous Asgari, the Iranian scientist, was ever part of a prisoner exchange. Since November, the 59-year old professor had been in the custody of Immigration and Customs Enforcement, or ICE, where he also contracted the coronavirus as he awaited deportation after a federal judge dismissed espionage charges against him.

In a statement to ABC News, State Department spokesperson Morgan Ortagus said Tuesday that Asgari "is not and has never been a participant in any prisoner swap with Iran."

Iran's Foreign Ministry also denied there was an exchange, adding during a briefing on Tuesday that Asgari "was accused of baseless theft of classified trade information, but the U.S. government failed to prove his guilt in court," according to the Iranian state media agency ISNA.

There had been back channel discussions in early May about an exchange, a source familiar with the talks told ABC News at the time. Speculation centered around the U.S. deporting Asgari and Iran allowing White, a cancer patient, to leave the country after his release on medical furlough.

A spokesperson for the White family told ABC News there has been no movement on his release and tweeted late Monday night that reports about his case "are not accurate. ... Please continue to pray for Michael's release."

In addition to White, the Iranian government has detained three dual Iranian American citizens -- Siamak Namazi, a businessman held since October 2015; his father Baquer Namazi, an 83-year old former United Nations Children's Fund (UNICEF) official and Iranian provincial governor; and Morad Tahbaz, an Iranian environmentalist with U.S. and British citizenship.

The family of former FBI agent Bob Levinson announced in March that U.S. officials believe Levinson, the longest-held American hostage who had been missing inside Iran since March 2007, died at some point in Iranian custody, but before the COVID-19 outbreak.

Like the U.S., Iran has been overwhelmed by its coronavirus outbreak, which has mushroomed through its prison system, in particular, and it now faces a possible second wave of infections. In March, Iranian authorities released nearly 100,000 prisoners on temporary leave to stem the virus' spread, but none of the detained Americans were on that list -- even as their prison floors saw the spread of COVID-19 cases, according to the Namazi's lawyer.

Last November, a federal judge dismissed the charges against Asgari, saying prosecutors had not provided enough evidence that he stole trade secrets while working at Case Western Reserve University. With his U.S. visa no longer valid, he was then detained by ICE for several months.

Ortagus said that the U.S. had been trying to deport Asgari since December, "but the Iranian government repeatedly has held up the process." An ICE spokesperson told ABC News on May 5 that his removal had been postponed because of the coronavirus pandemic.

Asgari contracted COVID-19 while in U.S. custody, according to his family and his lawyer. The ICE spokesperson said only that they expected "to effect his removal when he is medically cleared to travel and normal air travel resumes," in a statement on May 5.

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