Washington - Retired FBI agent Robert Levinson on Tuesday became the longest-held hostage in US history, with his family and US officials appealing to the new Iranian leadership for help in finding and releasing him.
Mystery shrouds the fate of Levinson, who disappeared on 9 March 2007 on Iran's Gulf island of Kish while reportedly investigating cigarette counterfeiting in the region.
"To whoever is holding Bob, I ask again for your mercy. Please let him go to reunite with his family," his wife Christine wrote on the website HelpBobLevinson.com set up by the family to press for his release.
"No one would have predicted this terrible moment more than six and a half years ago when Bob disappeared. Our family will soon gather for our seventh thanksgiving without Bob, and the pain will be almost impossible to bear," she added.
But in her message she urged her husband to stay strong and said he has a new month-old grandson.
"We can't wait for you to meet him. We love you and will never stop working to bring you home safely."
Levinson has now been held longer than the former AP Beirut bureau chief Terry Anderson, who was kidnapped in Lebanon in 1985 and held for 2 454 days by Hezbollah militants.
Virtually no news has filtered out about Levinson, now aged 65, since his capture other than a hostage video of him received by the family three years ago and a few pictures sent to the family in 2011.
The photos show him shackled in what appears to be an orange jumpsuit with a shaggy, gray beard and unkempt hair, holding a cryptic sign saying "4th year. You can’t or you don't want?"
The ongoing detente between Iran and the United States, however, has raised the family's hopes that the new Iranian government of President Hassan Rouhani may be prepared to help win his release.
"Given the negotiations between the United States and Iran over Tehran's nuclear programme, we particularly hope that officials can use their ongoing contact to resolve my father's case," Dan Levinson writes on the website.
"Doing so would show the world that our two countries can work together to resolve our differences and would demonstrate Iran's willingness to help an average American family's plight."
In a statement on Tuesday the White House renewed its commitment to seeking Levinson's safe return.
"We respectfully ask the government of the Islamic Republic of Iran to assist us in securing Mr Levinson's health, welfare, and safe return," the statement added.
While the recent talks in Geneva between world powers and Iran focused solely on the Iranian nuclear programme, State Department spokesperson Jen Psaki said the US had "repeatedly" raised Levinson's case in bilateral talks.
President Barack Obama had even mentioned him, along with other detained Americans Amir Hekmati and Saeed Abedini during his landmark phone call with Rouhani in September, she added.