Robert Levinson, a former FBI agent who went missing in Iran in 2007, "may have passed away some time ago," according to U.S. intelligence officials. The statement came after Levinson's family released a statement of their own on Wednesday, saying they were told by U.S. officials Levinson was most likely dead.

"As President Trump said today, Iran must provide a complete accounting of what occurred with Bob Levinson before the United States can fully accept what happened in this case," National Security Advisor Robert C. O'Brien told CBS News in a statement. "While the investigation is ongoing, we believe that Bob Levinson may have passed away some time ago. The United States calls upon Iran to release all Americans who remain wrongfully detained in that country. The safe return of all Americans held hostage or wrongfully detained overseas remains a top priority of the Trump Administration."

In their statement, the Levinson family wrote that "We recently received information from U.S. officials that has led both them and us to conclude that our wonderful husband and father died while in Iranian custody."

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"We don't know when or how he died, only that it was prior to the COVID-19 pandemic," the family added.

A spokesperson for Iran's mission at the U.N. insisted on Twitter Thursday that Tehran doesn't know where Levinson is and that he's not in Iranian custody, the Reuters news agency reports.

Later Thursday, a spokesperson for Iran's Foreign Ministry said there was "credible evidence" Levinson left the country years ago for an unknown location, according to Reuters. He added that Tehran has tried to learn what condition Levinson was in but hasn't been able to find "any signs of him being alive."

The former FBI agent was on an unauthorized CIA mission in Iran when he went missing. Since his disappearance, the official U.S. stance had been that he was being held by the Iranian government. As recently as March of last year, Secretary of State Mike Pompeo was calling on Iran to release Levinson.

Pompeo said shortly before the 12 year anniversary of Levinson's disappearance that the U.S. government remained "unwavering in our commitment to reunite the Levinson family with their beloved husband and father, who served our great nation during a long and distinguished career."

Retired FBI agent Robert Levinson seen captive in this undated photograph. Levinson family

When asked about Levinson during a White House Coronavirus Task Force briefing on Wednesday, President Trump stated, "It's not looking promising."

"I won't accept that he's dead," Mr. Trump added. "They haven't told us that he's dead. But a lot of people are thinking that is the case."

Levinson's family had held out hope he was still alive when Iran released a photo of Levinson in April 2011. In November 2019, Iran told the UN that Levinson's case was "ongoing," which gave the family hope he was still alive.

"It shows exactly what we've believed all along, which is that the Iranians took him back in March of 2007 and that they have the ability to send him home to us," Levinson's daughter told CBS News last year.

"The FBI Agents Association released a statement Wednesday expressing, "its deepest sympathies to the family of former FBI Agent Robert Levinson."

"The courage that the Levinson family showed over the past 13 years in their continued efforts to raise awareness about Bob's disappearance and to bring him home has been an inspiration to all, including his former colleagues at the FBI," the statement said. "We will not forget Bob and will support all efforts to bring to justice those responsible for this horrific crime. We continue to stand with the Levinson family during this difficult time."