Trump last year claimed that he’d gotten 17 people out of imprisonment around the world. “We’re very proud of that record. Very proud,” he said then. “And we have others coming.” In most cases, the U.S. determined the detentions to be unjust and arbitrary—like, for instance, the case of Aya Hijazi, an Egyptian American aid worker who was imprisoned in Egypt in 2014. In 2017, the Trump administration negotiated her release and brought her to the Oval Office to celebrate. Another case—that of the American college student Otto Warmbier—ended tragically; the Trump administration managed to secure his release and that of three others imprisoned in North Korea, but by the time Warmbier got home, he was suffering such serious injuries from his captivity that he died within days. He arrived home in a coma and never woke up.

Read: The Americans left behind in Iran

With some exceptions, the U.S. has generally articulated a policy of no exchanges or ransoms for prisoners, for fear of encouraging further hostage-taking. Asked about Americans imprisoned in Iran, U.S. officials have tended to insist that Iran should unconditionally release the prisoners because it’s the right thing to do, and have refused to discuss inducements such as sanctions relief.

Yet some cases get tangled up with other objectives and other countries’ policies. In November, for instance, the U.S. facilitated a deal in which the Afghan government swapped three Taliban prisoners for the freedom of two Western university professors in Taliban captivity since 2016, one of them American. Within weeks, the U.S. formally restarted peace talks with the Taliban after breaking them off in September.

And when Barack Obama’s administration secured the release of six U.S. citizens from Iranian imprisonment in 2016, it was part of a swap for seven Iranians in the United States. Wang will come home to the United States as Masoud Soleimani, an Iranian stem-cell scientist held on sanctions-evasion charges, goes free and heads back to Iran.

Wang’s release still leaves at least five other Americans detained or unaccounted for in Iran, including the businessman Siamak Namazi and his elderly father, Baquer. Siamak Namazi’s brother Babak said in a statement today that he was “absolutely thrilled” for Wang and his family. “At the same time,” he wrote, “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.” Siamak Namazi, who was detained in October 2015, was not part of the prisoner exchange the Obama administration implemented in January 2016; news of his father’s arrest came the following month.

Robert Levinson, who disappeared from Iran in 2007 while on an unofficial mission for the CIA, has now been gone for more than a decade, and his whereabouts are a mystery. The Iranian government has denied holding him and the last proof of life appeared in a 2010 video, in which, according to the Associated Press, Pashto music could be heard in the background, suggesting that he might have no longer been in Iran but in Afghanistan or Pakistan.