The family of Robert Levinson, a former FBI agent held for nearly 13 years in Iran, said on Saturday that they are happy about the release of American student Xiyue Wang in a prisoner exchange, but can't help feeling "extremely disappointed" that their husband and father continues to be imprisoned.

The Levinsons said they send their best wishes to Wang's wife and his young son: "This is a day they have long hoped for, but this news is bittersweet for our family."

Levinson has been held hostage longer than any other American, the family's statement noted.

"We can't help but be extremely disappointed that, despite all its efforts, the United States government was unable to secure his release," the statement said. "Iranian authorities continue to play a cruel game with our father’s life, and with our family. But the world knows the truth, and Iranian leadership must come clean. It is time for Iran to send Bob Levinson home, so he can live the rest of his life in peace."

Wang, 38, was released and able to head home on Saturday after spending more than three years in an Iranian prison. The U.S. and Iran negotiated in Switzerland to exchange Wang for Iranian citizen Massoud Soleimani, who was being held in an Atlanta jail over accusations that he had violated U.S. sanctions.

Levinson, 71, was working with the CIA on an unauthorized intelligence-gathering mission when he disappeared in March 2007 on Kish Island, a resort area off Iran's coast. He is now the longest-held American hostage in history, but his condition and whereabouts are largely unknown.

Iran acknowledged in a filing to the United Nations last month that it had an open case against Levinson.

Otherwise nothing has been known since family members released photos and video of him in 2011 that included a request for help and a warning that he was running out of medication to treat his diabetes.

Secretary of State Mike Pompeo tweeted Saturday that the U.S. is committed to bringing home every American held by Iran and other countries.

We will not rest until we bring every American detained in Iran and around the world back home to their loved ones. We thank the Swiss government for facilitating the return of Mr. Wang, and are pleased the Iranian government has been constructive in this matter. (2/2) — Secretary Pompeo (@SecPompeo) December 7, 2019

Stephanie Levinson Curry, the daughter of the former FBI agent, told Fox News in an interview last month that she felt encouraged by the work that President Donald Trump had done to ensure his return, including an offer of a $25 million reward for information about the prisoner.

"Our family is not political, but we think that President Trump is demonstrating his leadership and showing his commitment to bringing hostages home," Curry said.

Curry's brother had sharper words for the Obama administration when it negotiated a complex prisoner exchange with Iran in 2016 that brought four Americans home, but not Levinson.

"Don't get me wrong. We're very happy for these families. But we wish we were among them," Daniel Levinson told MSNBC's Andrea Mitchell at the time, noting his view that the Obama administration had failed his family.

"We're not getting any answers," he added. "We have been abandoned. It's the worst feeling in the world."

Obama administration officials at the time said they had not been able to determine Levinson's whereabouts, but had repeatedly brought up his disappearance with Iranian officials.

Iran, in response, denied any knowledge of Levinson's location in 2016 and offered to help search for him.

The former FBI agent isn't the only American currently being held by Iran. Iranian-American father and son Siamak and Baquer Namazi and U.S. Navy veteran Michael White remain imprisoned there.

Babak Namazi, the son of Baquer Namazi and brother of Siamak Namazi, said he was also excited for Wang's release but that his family was still waiting for answers.

"I am beyond devastated that a second President has left my ailing father Baquer Namazi and brother Siamak Namazi behind as American hostages in Iran in a second swap deal," Babak Namazi said. "I hope, pray, and expect that this is not a one-time trade but the beginning of an expedited process that will bring my family home soon."