There are numerous theories about why Iran keeps jailing Iranian-Americans, including hard-line efforts to undercut the administration of President Hassan Rouhani. Another motive, however, has been clear at least since the days of Rouhani's predecessor: seeking to trade detainees for Iranians in US custody.

Last week, the family of Amir Hekmati, an Iranian-American ex-Marine imprisoned in Iran since visiting relatives there in 2011, released a letter sent by Hekmati to Rouhani. “For the past three years, my family has been receiving emails and phone calls from individuals in Iran proposing prisoner exchanges, going so far as asking my family to lobby publicly for the release of these individuals,” the letter said.

A spokesman for the Hekmatis would not divulge the names of these Iranian individuals. Asked for a list of current Iranian prisoners in the United States and whether Iran was demanding their freedom in exchange for the detainees in Iran, a spokesman for Iran’s UN mission did not respond.

Iran in the past has sought the return of Ali Reza Asgari, a retired Islamic Revolutionary Guards Corps general who disappeared in Turkey in 2006 and is said to have defected to the United States. Another Iranian of interest to Tehran, merchant Mohsen Afrasiabi, disappeared in Germany several years ago.