Story highlights Retired FBI Special Agent Robert Levinson vanished in 2007

He was working as a private investigator

The U.S. "reiterates its call on the Government of the Islamic Republic of Iran," official says

The FBI is offering $1 million for information leading to Levinson's safe return

Wednesday marks day 2,000 since retired FBI agent Robert Levinson disappeared in Iran, and the State Department is urging Tehran to provide information on his whereabouts.

"Determining Mr. Levinson's whereabouts and reuniting him safely with his family continues to be a priority for the U.S. government. The United States also continues to welcome the assistance of our international partners in this investigation," U.S. State Department spokeswoman Victoria Nuland said.

An FBI reward of $1 million for information leading to Levinson's safe return still stands.

Levinson, who became a private investigator after retiring from the FBI, was working on a cigarette smuggling case when he disappeared more than five years ago while on a business trip on Iran's Kish island on March 8, 2007.

The State Department has consistently denied Levinson was working for the U.S. government and has unsuccessfully pressed Tehran for information.

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In March 2011, Secretary of State Hillary Clinton announced the U.S. government had received indications that Levinson was being held by a group in a region that includes the border areas of Afghanistan, Iran, and Pakistan. At the time, the Iranian government said it would make inquiries about Levinson but did not know what happened to him.

Later, the FBI said it did not know what group was holding Levinson, what the group's demands might be or even a reason for his capture.

Whoever his captors are, they sent a "proof of life" videotape and photographs of Levinson to his family in 2010.

In the video, Levinson asked the U.S. government for help: "Please help me get home," said a gaunt-looking Levinson on the tape, citing his 33 years of service in the FBI as a reason for that assistance.

Levinson suffers from diabetes and appeared to have lost 50 to 60 pounds since his capture. He turned 64 on March 10. In addition to his wife of 38 years, Levinson has seven children and two grandchildren.

Levinson's family aired their angst and anguish in a statement released Tuesday:

"Two thousand days since Bob has heard the voices of his children and his wife, and since we have heard his. Two thousand days since he has worn the clothes in his closet -- which we are sure no longer fit him - and 2,000 days since he has slept on his side of the bed.

"It is time for Bob to be released."