Levinson Family via AP This undated photo shows retired-FBI agent Robert Levinson, who went missing on the Iranian island of Kish in March 2007. Levinson's family received this and other photographs of him in April 2011.

Robert Levinson, the former FBI agent who was apparently kidnapped during a business trip to Iran six years ago, marked a sad milestone Tuesday. He’s now been held captive for 2,454 days, longer than Associated Press bureau chief Terry Anderson, who was held in Beirut from 1985 to 1991, making Levinson one of the longest held American hostages in a foreign land.

But U.S. officials, including President Obama, are giving Levinson’s mysterious case fresh attention, in hopes that the new government in Tehran will help secure his release.

Iranian officials have repeatedly denied any knowledge of what happened to Levinson, 65, who disappeared after checking out of his hotel on Kish Island on March 9, 2007. Levinson, who was in Iran working as a private investigator for a British tobacco company, had previously served for 28 years in the FBI and DEA and was known as an expert on Russian organized crime.

U.S. officials, however, are now prodding the new Iranian administration of President Hassan Rouhani to be more cooperative and expressing cautious optimism that his government will prove helpful, especially in light of the recently signed interim nuclear accord.

The White House said in a statement Tuesday that U.S. officials have raised Levinson's case -- as well as those of two other Americans being held captive in Iran, Saed Abedini and Amir Hekmati -- in recent talks with Iranian officials and “we will continue to do so."

Officials also confirmed that President Obama personally brought up the cases when he spoke to Rouhani in September. “We want to see these Americans reunited with their families,” a White House spokeswoman said Tuesday.

Jalil Rezayee / EPA Afghans look at a billboard promising a reward for information leading to the recovery of former FBI agent Robert Levinson, in Herat, near the Iranian border, on April 24, 2012.

In a column published Tuesday in the Washington Post, the former agent’s son, Dan Levinson, said it was “not possible to overstate the nightmare that the past 6 years 8 months have been,” and asked the Rouhani administration for help. "I'm a lot more optimistic," he said in an interview.

"Hopefully, this is a new opportunity," said Jacqueline Maguire, a spokeswoman for the FBI, which is making its own efforts to publicize Levinon's case.

The agency, which last year offered a $1 million reward for information about Levinson, released a new statement on its website Monday noting how long Levinson had been held.

"Exhaustive efforts have not yet been successful in locating Bob or establishing a dialogue with those who are holding him,” said FBI Director James Comey in the statement, “but the FBI remains wholly committed to bringing him home safely to his loved ones."

The bureau has a task force assigned to the Levinson case, and recently ran advertisements with photographs of Levinson in three U.S.-based Farsi-language publications. It also put up billboards seeking information about Levinson in Afghanistan as well as in mid-town New York during last year's General Assembly.

Although the bureau has gotten some leads, "nothing has panned out," said Maguire.

In a statement to NBC News, an Iranian government spokesman once again denied having any information about Levinson.

“Iran has had maximum cooperation with Mr. Levinson's family and his family had a trip to Iran,” said Alireza Miryousefi, counselor to Iran’s mission to the United Nations. “Iranian organizations had worked with his family to find out where is Mr. Levinson. We are interested too to find out why Mr. Levinson was in Kish."

Miryousefi added, "As it was said before, we have no information about Mr. Levinson and his location."

Levinson had gone to Iran as a private investigator working for British American Tobacco, which had hired him to probe cigarette smuggling, according to Dan Levinson.

In 2010, the family received a video showing Levinson in captivity, and in 2011 it received via email photographs of a bearded Levinson in what appeared to be an orange prisoner jump suit. The email with the photographs was traced to a Pakistani IP address, but neither the family nor the U.S. government has gotten any further information about his location or who is holding him.

There are multiple theories about who might be holding Levinson, ranging from Iran's Revolutionary Guard or an Iranian political faction to a rogue terrorist group. But FBI officials acknowledge they have no hard evidence about who it might be.

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