By CNN Foreign Affairs Correspondent Jill Dougherty
With a breakthrough interim nuclear deal and relations between the United States and Iran improving, the White House on Tuesday "respectfully" asked the Iranian government to help return Robert Levinson, a retired FBI agent who went missing in Iran more than six years ago.
"We do want to test the regime. The new administration has said that they want to take a different approach toward the West, toward the United States. One way that they could clearly demonstrate that is they could help us find Bob Levinson, help reunite him with his family," Deputy National Security Adviser Ben Rhodes told CNN.
Rhodes said that Levinson and two other Americans are being "detained in Iran unjustly, in our view."
A separate White House statement said the United States welcomes help from "our international partners in this investigation, and we respectfully ask the government of the Islamic Republic of Iran to assist us in securing Mr. Levinson's health, welfare, and safe return."
Levinson became a private investigator after his three-decade career with the FBI and disappeared in March 2007 during a business trip to Kish Island, Iran. It is believed he was investigating cigarette smuggling.
His wife, Christine, told CNN the family has no new information about him but is "hopeful," with the leadership of Iran's new and more moderate president, Hassan Rouhani.
Christine Levinson said Tuesday marked "an unimaginable milestone" for her husband, noting he has now surpassed journalist Terry Anderson to become the "longest-held American hostage." Anderson spent 2,454 days in captivity before being freed by Islamic militants in Lebanon in 1991.
"Our family will soon gather for our seventh Thanksgiving without Bob, and the pain will be almost impossible to bear," she said in a statement.
Iran's government repeatedly has said it is not holding Levinson and does not know his whereabouts.
Iran's previous president, Mahmoud Ahmadinejad, said he was willing to help find Levinson, and the family received what then-Secretary of State Hillary Clinton described in 2010 as "proof of life."
CNN was told some discussions with Iran were being held. The White House said there still is uncertainty over Levinson's whereabouts.
FBI Director James Comey said Monday that the agency continues "to follow every lead into his disappearance."
CNN's Susan Candiotti contributed to this report.