Kidnapped American Robert Levinson’s son Daniel penned an op-ed for the Washington Post over the weekend, in which he warned those who would visit Iran in pursuit of business opportunities created by the nuclear deal to exercise extreme caution. “My family and I cannot emphasize enough how dangerous traveling to Iran remains,” Daniel Levinson wrote.

Eight years ago, retired FBI agent Robert Levinson disappeared in Iran while working as a CIA contractor, keeping tabs on the Iranian nuclear program and the terrorist group Hezbollah. Although photos of him in a prison jumpsuit have surfaced as recently as 2013 – he held a sign reading “Help me” in one of the pictures – the Iranian government insists it had nothing to do with his disappearance, and has no idea where he is. He has now been held longer than any other American in captivity, despite the FBI posting a $5 million reward for his safe return.

In addition to his father, Daniel Levinson lists other detainees: “Iran is holding four other U.S. citizens, including Post reporter Jason Rezaian. It temporarily detained 15 members of the British navy two weeks after my father’s detention and several U.S. and European citizens in the years since.”

“Any foreign national considering a trip to Iranian-controlled territory risks arbitrary detention, potentially without access to any basic human rights or their loved ones for years to come,” Levinson concludes. “This is what happened to my father.”

He takes pains to emphasize that he and his family were treated well when they visited Iran to search for Robert Levinson in 2007, and said ordinary Iranians displayed “kindness” and “sympathy.” He is also in favor of maintaining communications between Washington and Tehran, and says the Levinson family was “optimistic” about the Iranian nuclear deal, only to be “devastated” when Iran’s prisoners were not released as part of the agreement. Years of the U.S. government pretending Robert Levinson was abducted by some random group for inexplicable reasons, and it would be awfully nice of Tehran to help find him, yielded no results.

“Now we fear that the United States has squandered its best opportunity for leverage in ensuring my father’s safe return home,” Levinson wrote, arguing that Iran will never release his father without strong “incentives” to do so.

He rather gently holds President Obama to task for failing to insist on the return of detainees as part of the nuclear deal, rendering his sharpest criticism by noting that after the president said he would not be “content” to “celebrate [the nuclear deal] with American citizens languishing in Iranian jails,” the Democrats went ahead and threw a party.

Levinson hopefully suggests that releasing his father and other captives might be in Iran’s best interests, as it would reassure foreign business and political partners that traveling to Iran, or establishing a business there, is safe. That is a good point, but it must surely have occurred to the Iranians already, and instead of showing leniency, they doubled down with such measures as sentencing the Washington Post’s Rezaian to prison in an outrageous secret trial.

The Iranians seem more interested in asserting dominance, and working out their nasty internal power struggles, than reassuring foreign partners about the safety of Tehran. They also do not seem to think the nuclear deal requires much in the way of meaningful concessions from Iran; they are not exactly leaping to fulfill the express terms of the deal, never mind throw in sweeteners Obama never asked for, such as releasing their hostages.

“A retired FBI agent and CIA contractor, my father spent over three decades of his life serving the United States: taking on organized-crime families in New York, keeping drugs off our streets and preventing the spread of the Russian mafia to our shores, among other accomplishments in his heroic career,” Daniel Levinson wrote in tribute to his father. “He has languished in isolation for almost nine years, living a nightmare away from everyone he knows and loves. The United States cannot leave one of its own behind, especially after he has given so much to this country.”