When the US and Iran carried out a much-publicised prisoner swap in January ahead of the signing of the P5 + 1 nuclear deal ending almost 40 years of Iranian isolation, Christine Levinson was hoping that her husband, Bob, 67, would be among the Americans released by the Islamic Republic. She was to be disappointed.

Robert Levinson, a former FBI agent, disappeared while on a business trip to the Iranian island of Kish in 2007 but Iran has always claimed that it does not know what happened to him. The family has released a new appeal for information about Levinson on the nine-year anniversary of his disappearance on March 9 and a day before he turns 68.

The White House continues to maintain the pressure on Iran and is said to be encouraged by Tehran's recent co-operation on wider geopolitical issues, such as nuclear talks between the two. A statement from US Secretary of State John Kerry, published today (March 9) on the US State Department website, adds: "For almost a decade, a beloved husband, brother, father, and grandfather has been kept from celebrating family milestones most take for granted. No one should have to endure what Mr Levinson and his family have endured for so long.

"The US government in its entirety will continue all efforts to locate Bob and bring him home. The Government of the Islamic Republic of Iran has committed to cooperating with the United States to determine the whereabouts of Mr Levinson, and we are holding Iran to its promise."

The FBI has even launched a Facebook page in Farsi to help track down the US citizen's whereabouts.

In 2010 and 2011 his family received a series of images of Levinson, then 64, in an orange jump suit and with a long grey beard. He was holding two poorly-spelled, hand-written signs that made reference to Guantanamo Bay and the US treatment of inmates there.

At the time, Mahmoud Ahmadinejad was president of Iran and relations between the US and Tehran were at historic lows. Although Iran has never admitted knowing where Levinson is, it was believed that the country's intelligence services were behind the kidnapping and the pictures.

In 2012, the FBI announced a $1 million reward for information leading to the recovery of the American. This sum was increased to $5 million in March last year. Then in 2013 his family announced that he had been working as a "freelance spy" for the CIA. This admission has never been confirmed by the US government.

But now moderate Hasan Rouhani leads Iran and the country appears to be on a path towards normalisation of its relations with the US. Despite the ending of sanctions, however, the Levinson family are no closer to figuring out where Robert is or even whether the Iranian authorities are holding him. On March 6, they published a new appeal for information about his disappearance on YouTube.

"I thought it would be a matter of days, and now it's been nine years," Christine told ABC News Tuesday. "Some of my children call me at midnight or later, crying, because they can't stop thinking about Bob. They don't know what to do. [They call] more so now, nine years later, in some cases than they did before.

"We have never, since the video or the pictures, received any information about what has to be done to get Bob home. We need proof of life and we need to know that he's okay, or we need them to let us know that he's not okay," she said.

In the YouTube video, Robert is seen surrounded by family and friends as a narrator says: "We need more than just a promise of help from Iran. We need answers. We need Bob Levinson sent home. We will never stop. Help Bob Levinson."

In a WikiLeaks cable between Tehran and the Vatican (with which it has always retained diplomatic relations) in 2008, Iran explicitly denied knowing anything about his disappearance.