Bob Levinson, the former FBI agent who went missing 13 years ago during an investigation into high-level Iranian corruption, is believed to have died, his family said on Wednesday.
The family of Levinson, 71, said they had received information from US officials that he died in Iranian custody.
He was last known to have been alive in 2011 when a video released to his family showed him urging the US government to help him return home.
Later pictures showed Levinson in an orange jumpsuit in what was regarded as a deliberate echo of conditions at Guantanamo Bay, the US detention facility.
He went missing during an investigation into the skimming of Iranian oil profits.
A US judge ruled this month that Levinson had been held and tortured by officials acting on behalf of the Iranian state.
“We don’t know when or how he died, only that it was prior to the Covid-19 pandemic,” his family said.
“It is impossible to describe our pain. Our family will spend the rest of our lives without the most amazing man we have ever known, a new reality that is inconceivable to us.”
The private investigator travelled to the Iranian resort of Kish Island to question a contact over claims that Iranian officials were taking a cut from oil sales and hiding the money in overseas investments.
Levinson checked out of his hotel after the meeting but has not been seen since March 9, 2007.
A video and photos received by his family suggested he was being held by an unidentified terrorist group but the US judge ruled Iran was responsible.
The family said they would campaign for the return of his body and for Tehran to be held accountable for what had happened.
“How those responsible in Iran could do this to a human being, while repeatedly lying to the world all this time, is incomprehensible to us,” they said.
“They kidnapped a foreign citizen and denied him any basic human rights, and his blood is on their hands.
“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.
“We will spend the rest of our lives making sure of this and the Iranian regime must know we will not be going away.
“We expect American officials, as well as officials around the world, to continue to press Iran to seek Bob’s return, and to ensure those Iranian officials involved are held accountable.”
On Thursday, Tehran claimed Levinson left Iran "years ago" without giving details of where he went.
"In the past years, Iran has tried to find out his state but could not find any signs of him being alive," Iran's Foreign Ministry spokesman Abbas Mousavi said, according to state TV.
US President Donald Trump called Levinson "a gentleman" and "outstanding".
"It's not looking good; he wasn't well for years anyway, in Iran," Mr Trump said. "It's not looking promising ... we feel terribly for the family."
"It's not looking great, but I won't accept that he’s dead.
"They haven't told us that he's dead, but a lot of people are thinking that that’s the case."
US National Security Adviser Robert O’Brien called on Tehran to produce evidence of what happened to Levinson.
“Iran must provide a complete accounting of what occurred with Bob Levinson before the United States can fully accept what happened in this case," Mr O'Brien said.
AP reported in 2013 that Levinson had been on an unauthorised CIA mission to investigate the Iranian government when he was detained.
Levinson’s wife, Christine, and his seven children gave evidence last year in a US court, seeking compensation from the Iranian government for unlawful hostage taking and torture.
They received strong backing in their campaign from the Trump administration.
The US had repeatedly called for his release and rewards of $25 million (Dh91.8m) was offered for his return.
The US Congress is preparing to debate a new law in Levinson's name, which is designed to help hostages and punish those responsible for holding them.