The family of Robert Levinson, the former Federal Bureau of Investigation agent who disappeared in March 2007 after traveling to an island controlled by Iran, said on Wednesday it now believes Levinson died in Iranian custody, based on information from US officials.
Mr Levinson disappeared in Iran's Kish Island in 2007 while on an unauthorised Central Intelligence Agency mission.
At a White House briefing on the coronavirus, Trump resisted confirming the family's account, saying that Iranian officials had not told the US about Levinson's fate and that "I won't accept that he's dead".
"Those who are responsible for what happened to Bob Levinson, including those in the United States government who for many years repeatedly left him behind, will ultimately receive justice for what they have done", the family's statement said.
On March 9, the FBI's Washington field office noted the anniversary of Levinson's disappearance and called the date "another opportunity for the leadership of the government of Iran to finally return Bob home to his family".
Iran's foreign ministry said in 2018 that Levinson had "travelled to Iran at a certain point and then left" and that it had no information about anything that might have happened after he left.
There are conflicting claims, but little hard evidence as to the circumstances surrounding Mr Levinson's disappearance.
The only known traces of Levinson surfaced in 2011 when family members released still photos and a video in which he pleaded for help. It did not say who was holding him or where.
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.
Iranian officials have repeatedly said they had no information about him. "Those facts have not changed", Miryousefi said in a tweet. "They will only know him through the stories we tell them".
However, his national security adviser, Robert O'Brien, said he believed Levinson "may have passed away some time ago".
In a statement on Wednesday, the Levinson family said it had recently received information from USA officials that had led them to draw their conclusion. "We don't know when or how he died, only that it was prior to the COVID-19 pandemic", they said in a statement. Nor did they know if Levinson's body would ever be returned to them.
The household mentioned Levinson died earlier than the coronavirus pandemic, which has hit Iran onerous and led authorities to launch hundreds of prisoners briefly. There he met with Daoud Salahuddin, an American "militant" who fled to Iran while facing charges in the murder of an Iranian embassy official based in Washington, DC.
Following the family's announcement and before O'Brien's comments, President Donald Trump told reporters that "I won't accept that he's dead".
"The Iranian state-run Press TV reported that he was, quote, 'in the hands of Iranian security forces, ' " Levinson said. They were later forced out of the agency and several more were disciplined after an internal agency investigation.
Washington also maintains tough economic sanctions on Iran.
Iran continues to be holding twin U.S. nationals Siamak Namazi, who was convicted in 2016 of fees that embody espionage and collaboration with the USA authorities, his father Baquer and environmental skilled Morad Tahbaz.