Robert Levinson: The American, Former FBI Agent, 'Left Behind' in Iranian Prisoner Release

— -- As American families cautiously celebrated the reported release of their loved ones from Iran, one has been "devastated" by the news that a former FBI agent was apparently "left behind."

"We are happy for the other families. But once again, Bob Levinson has been left behind," the Levinson family said on Facebook. "We are devastated."

Family members said that Levinson was part of the discussion with the government of Iran that eventually freed five Americans in Iranian custody and also ended in the clemency for seven Iranians in the U.S., but the Iranians denied knowing where Levinson was.

The 67-year-old Levinson, who served for more than two decades with the FBI before retiring, disappeared off Iran's Kish Island in March 2007. For years the U.S. government said Levinson was working at the time as a private investigator, but in December 2013 his family acknowledged that he was, in fact, working as a freelance "spy" for a rogue CIA operation.

Family attorney David McGee told ABC News then that he felt the CIA and the FBI had betrayed Levinson and tried to hide the fact that he had a long-term relationship with the CIA, spying on Iran's nuclear program and on the Lebanese terror group Hezbollah.

"The CIA sent Bob Levinson to Iran to do an investigation on its behalf," McGee said. "[R]ather than acknowledge what they had done and try to save Bob's life, they denied him."

Amid the news about the other Americans' release today, a U.S. official told ABC News, "Iran has also committed to continue cooperating with the United States to determine the whereabouts of Robert Levinson."

Iran has long denied it is holding Levinson, but U.S. officials previously have told ABC News they suspect that at the very least, officials in the Islamic Republic know where he is. Some U.S. officials told ABC News today that Levinson being left out of the highly-publicized prisoner release raises fears that he may not still be alive.

In January 2013, Levinson's family released a series of images of the then-64-year-old man, showing him draped in chains and dressed in what appears to be a mock prison uniform. In each he holds a sign, one of which reads "Help me." American authorities either do not know or have not publicly identified Levinson's suspected captors, but the U.S. government has repeatedly asked the Iranian government's help in finding him.

“Bob’s former colleagues have not forgotten him. We continue to stand with the Levinson family,” said FBI Agents Association President Renaldo Tariche. “Bob, married for 38 years with seven children, has missed more than nine years of accomplishments and milestones in the lives of his children. Each day brings renewed heartache to his family and friends. We celebrate today’s release of Americans, but the world should not forget Levinson’s continued unjustified imprisonment and continue to work for his release.”