WASHINGTON — Iran and the U.S. came to the brink of war in January but the two sides are still keeping open a diplomatic channel to discuss the fate of Americans imprisoned in Iran, according to multiple sources familiar with the talks.

The communication is indirect, with messages passed through the Swiss government, which has handled U.S. interests in Tehran since the U.S. and Iran severed diplomatic relations 40 years ago.

The Swiss ambassador to Iran, Markus Leitner, has made frequent trips to Washington in recent months to relay information on the prisoners’ status and messages from Tehran, a European diplomat and two sources familiar with the matter told NBC News.

Private individuals working on behalf of the relatives of those held in Iran, including former New Mexico Gov. Bill Richardson and his associates, are also engaged in discussions with Iran in hopes of securing the release of U.S. citizens in Iranian prisons.

“We are in constant contact and are trying to find a way to get Americans back home,” Richardson told NBC News.

The families of Michael White, a U.S. Navy veteran arrested in 2018, and Robert Levinson, a former FBI agent who went missing in Iran in 2007, have asked Richardson and his team to help with their cases. But Richardson said that “the tense state of relations between the two countries is not making things easy.”

The State Department declined to comment when asked about indirect talks via the Swiss. The Swiss embassy declined to comment.

In December, Iran released Xiyue Wang, a Princeton graduate student held for three-and-a-half years, as part of a prisoner swap in Switzerland. The exchange raised the possibility of a potential opening or at least a reduction of tensions between two governments that have traded threats for months and teetered on the verge of military conflict.

“Thank you to Iran on a very fair negotiation. See, we can make a deal together!” President Donald Trump tweeted shortly after Wang was released at the Zurich international airport.

Richardson said Wang’s release was designed to be part of a series of releases and humanitarian steps over time. “We’ve been working with the Iranians, from the start, to have a sequence of mutual gestures including further releases. Wang’s release was the first. This was all about getting a positive momentum based on humanitarian gestures.”

But weeks later, an American contractor was killed in northern Iran in a rocket attack that the Trump administration blamed on Iranian-backed militia. That attack set off a chain of events culminating in a U.S. drone strike that took out Iran’s top general Qassem Soleimani in Baghdad, one of the country’s most powerful figures. Iran retaliated by launching ballistic missiles at Iraqi bases housing U.S. troops.

The military confrontation delivered a serious setback to attempts to resolve the cases of the remaining Americans held in Iran, the sources said.

“That was a huge blow for our efforts,” said one source familiar with Richardson’s work.

Four Americans in Iran

There are at least four Americans known to be held in Iran: Michael White, a Navy veteran arrested in 2018; Morad Tahbaz, an environmental activist detained in 201; Siamak Namazi, an Iranian-American business consultant arrested in October 2015; and his father, Baquer Namazi, a former senior UNICEF official arrested in 2016 and later placed on medical furlough in Tehran. Former FBI agent Robert Levinson has been missing since a visit to Iran’s Kish Island in 2007 and the U.S. government has repeatedly demanded information about his whereabouts.

The U.S. and human rights groups consider the Americans imprisoned in Iran as hostages that have been arrested and convicted on baseless charges without fair trials. Iran rejects that description.

Michael White’s mother, Joanne White, told NBC News she is frustrated with how the Trump administration has approached her 48-year-old son’s case and is gravely concerned about his health.

“If it was a priority they would have brought him back already,” said White. “I don’t think they’re paying enough attention.”

White was arrested in 2018 after visiting a girlfriend in Iran he had met online.

Joanne White said her son has multiple chronic conditions, including cancer and asthma, and she was frightened his fragile health could not withstand long-term imprisonment. Before he left for Iran, her son had just finished his chemotherapy for neck cancer. During his imprisonment, she said, his chemotherapy port has not been properly flushed or cleaned, which could lead to a potential lethal blood infection.

“I need them to bring him back before it’s too late,” White said.

She said she was terrified for her son’s safety after the U.S. and Iran appeared on the verge of a shooting war last month. “It panicked me, because he’s in over prison there.”

Swiss diplomats have not been allowed to visit Michael White since August, according to a spokesperson for the family, Jonathan Franks.

Secretary of State Mike Pompeo cited White’s plight in a tweet marking the American’s birthday last month. “For the second straight year, U.S. Navy veteran Michael White spent his birthday suffering in a horrible Iranian prison. The Iranian regime must release all American and foreign hostages!”

In a statement, the State Department said, “Before and since the release of Xiyue Wang two months ago, the U.S. government has placed a high priority on the return of wrongfully detained U.S. citizens from Iran. We maintain ongoing contact with the family of the detainees and, in addition to our diplomatic efforts, regularly condemn publicly these troubling, prolonged detentions.”

Iran has denied White has been mistreated or deprived of adequate medical care. Iran’s mission to the United Nations did not respond to a request for comment.

Richardson vs. U.S. officials

The release of Wang in December showed the Trump administration was open to prisoner exchanges, something previous administrations have tended to rule out. But it also has led to some friction over who should claim credit for the release, with U.S. officials and Richardson in a dispute about what preceded the prisoner releases.

Wang’s release came about partly through more than two years of discreet talks by private intermediaries helping the Princeton graduate’s family, according to Richardson and his colleagues, and Jim Slattery, a former Democratic congressman from Kansas who has forged contacts with Iranian officials through an interfaith initiative.

Foreign Minister Mohammad Javad Zarif had called publicly in April last year for a comprehensive swap of Iranians imprisoned in the U.S. and all Americans held in Iran. As of November 2019, there were 29 Iranian citizens in U.S. custody, according to the U.S. Bureau of Prisons.

Zarif’s proposal was rejected by the administration, so Richardson and Slattery started trying to tackle one case at a time.

It soon became clear that Iran might be ready to release Wang if the U.S. released an Iranian scientist, Masoud Soleimani, specializing in stem-cell research, who was in prison in Atlanta after being accused of violating U.S. sanctions, according to Richardson’s team.

In consultation with administration officials, Slattery, Richardson and Wang’s lawyer, Jason Poblete, worked out a plan for Wang to be released at about the same time as Soleimani, whose case appeared to be wrapping up.

U.S. prosecutors pursued a plea bargain agreement with the Iranian’s scientist’s lawyers, which would allow him to be released based on the time he had already served awaiting trial. The plan for Soleimani’s release and Wang’s release advanced, with logistical arrangements discussed with Iran with the help of Qatar.

At the 11th hour, senior administration officials abandoned the Richardson/Slattery plan for the plea agreement and prosecutors dropped all charges against Soleimani, moving up his release by several days, according to Soleimani’s defense lawyer, Lenny Franco.

U.S. officials then called Franco and his colleague on a Thursday and told them they were going to do a prisoner exchange and it would take place in two days.

“It was fantastic,” Franco said. He said he had never seen anything like it in his career, but believed it was the best possible outcome for all concerned and didn’t press for a detailed explanation. “As a defense attorney, when a prosecutor offers to dismiss the case, you don’t need to know why.”

Brian Hook, special envoy on Iran at the State Department, was initially opposed to the prisoner exchange as he saw it as undermining the Trump administration’s maximum pressure policy, according to sources familiar with the talks. Hook’s view later shifted and he welcomed Wang after his release at Zurich’s international airport.

“We negotiated the deal and advanced it towards a set implementation date. We coordinated and shared information with the White House and the National Security Council,” Richardson said.

“Some players in the U.S. government then decided to cut us off just before implementation in order to claim credit.”

The administration strongly disagreed with Richardson’s portrayal.

“The State Department entirely rejects Governor Richardson’s baseless accusations,” a State Department spokesperson said.

The Justice Department declined to comment when asked why prosecutors decided to drop all charges against Soleimani.

The New Yorker first reported Richardson’s account of the prisoner exchange.

Slattery said it wasn’t worth arguing about who deserved credit as the exchange was a success. “All’s well that ends well, right?”

“Figuring out how to release Dr. Soleimani based on the merits of his case was the key to obtaining the release of Wang,” Slattery said. “I want to give credit to all who helped on the Wang /Soleimani case.”

He added that Iranian officials, including Foreign Minister Mohammad Javad Zarif and Iran’s Ambassador to the U.N. Majid Takht-Ravanchi, “kept their word to me.”