The FBI is offering a $1m (£636,000) reward for information leading to the safe return of missing former agent Robert Levinson, last seen in Iran.
This week is the fifth anniversary of Mr Levinson's disappearance.
The US believes he is being held in south-west Asia, and in 2010, video of Mr Levinson was sent to his family by his captors, the FBI says.
Mr Levinson was working as a private investigator when he went missing on Kish, an island in the Persian Gulf.
He retired from the agency as a special agent in 1998 after two decades of service.
"Though he is retired from the FBI, Bob remains a member of the FBI family to this day," Robert Mueller, the agency's director said in a statement announcing the reward.
'Home by now'
Iran's elite military force, the Revolutionary Guards, has denied reports it was holding the retired agent.
The country's foreign ministry has promised to help the search for Mr Levinson for "humanitarian" reasons.
Secretary of State Hillary Clinton said finding Mr Levinson remained "a priority for the United States" and that the state department welcomed "the assistance of our international partners in this investigation".
The reward will be coupled with a new public information campaign by the FBI across the region.
"I am very grateful that the FBI has offered this reward," Christine Levinson, the missing man's wife, said in a statement.
"We all thought Bob would be home by now. But five years have passed, and we still don't know why he's being held, who has him, or where he is."