The United States government has concluded that retired FBI agent Robert Levinson, who disappeared more than a decade ago, has died while in the custody of Iran, his family said Wednesday.

The family said in a statement posted on Twitter that it had no information about how or when Levinson had died, but that it occurred before the recent coronavirus outbreak. The family said information US officials had received led them to conclude that he is dead.

"It is impossible to describe our pain," the family's statement said. "Our family will spend the rest of our lives without the most amazing man, a new reality that is inconceivable to us. His grandchildren will never meet him. They will know him only through the stories we tell them."

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Levinson disappeared on March 9, 2007, when he was scheduled to meet a source on the Iranian island of Kish. For years, US officials would only say that Levinson was working independently on a private investigation. But a 2013 Associated Press investigation revealed that Levinson had been sent on a mission by CIA analysts who had no authority to run such an operation.

"Those who are responsible for what happened to Bob Levinson, including those in the US government, who for many years repeatedly left him behind, will ultimately receive justice for what they have done," the family statement said. US officials have not confirmed the conclusion cited in the family's statement.

The family received proof-of-life photos and a video in late 2010 and early 2011, but his whereabouts and fate were not known.

In November, the Iranian government unexpectedly responded to a United Nations query by saying that Levinson was the subject of an "open case" in Iranian Revolutionary Court. Though the development gave the family a burst of hope, Iran clarified that the "open case" was an investigation into his disappearance.

The announcement of his death comes just weeks after a federal judge in Washington, DC, held Iran liable for his disappearance, saying the country was "in no uncertain terms" responsible for Levinson's "hostage-taking and torture".

Iranian officials did not immediately comment on Levinson's family's statement.