U.S. authorities have increased their offer of a reward for information that could lead to the safe return of Robert Levinson, a former FBI agent and CIA contractor who vanished in Iran.

Marking the eighth anniversary of his disappearance, the FBI on Monday said it would offer a reward of $5 million, up from $1 million.

The whereabouts of Levinson, who turns 67 on Tuesday, are unknown, but U.S. intelligence officials suspect the Iranian government played a role in his abduction from the island of Kish. Iranian officials say they do not know who took Levinson.

Levinson went to Kish trying to collect information about the Iranian government. He was in the process of renewing his contract with the CIA when he was taken after supposedly checking out of his hotel.

It has been years since Levinson’s family received any indication that he is still alive.

This video frame grab shows retired FBI agent Robert Levinson in a video received by his family in November 2010. (Levinson family via Associated Press)

Levinson has now become one of the longest-held hostages in U.S. history, surpassing the 2,454 days that journalist Terry Anderson spent in captivity in Lebanon from 1985 until 1991.

“It is long past time for Bob to come home,” FBI Director James B. Comey said in a statement Monday.