An FBI poster displayed in Washington in 2012 shows retired agent Robert Levinson, who disappeared from Kish Island, Iran, on March 9, 2007. (Manuel Balce Ceneta/Associated Press)

Daniel Levinson and his family run the website and the Facebook page

American and European companies are drafting plans to begin doing business in Iran with the lifting of sanctions as part of this summer’s nuclear-weapons agreement, and Westerners are planning visits to the country. My family and I cannot emphasize enough how dangerous traveling to Iran remains.

It is widely known that my father, Robert Levinson, was detained on Iran’s Kish Island on March 9, 2007. Iranian state media even reported as much at the time, though Tehran now denies knowledge of his whereabouts. 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. This is what happened to my father.

My mother, my aunt and I went to Iran in late 2007 to retrace my father’s footsteps and meet with officials. We were treated well, and I was struck by the kindness of ordinary Iranians, their sympathy for our situation and the beauty of the country. I would love to return after my father’s case is resolved to see more of what Iran has to offer, but I couldn’t imagine doing so for “fun” anytime soon. We urge everyone to think twice before traveling there.

My family has always advocated maintaining an open line of communication between Washington and Tehran, as we believe it can pave the way to improved relations and progress on key issues. We were optimistic about President Obama’s pursuit of direct talks with Iranian officials. In particular, we saw the nuclear talks as a golden opportunity to resolve my father’s case, so long as both sides were willing to negotiate. However, we were devastated that he was not released in the aftermath of the accord. Now we fear that the United States has squandered its best opportunity for leverage in ensuring my father’s safe return home.

Of course, the Iranian government is ultimately responsible for my father’s suffering. If the country’s supreme leader, Ayatollah Ali Khamenei, said four words — “Send Robert Levinson home” — this nightmare would end. But as is the case in matters of diplomacy, we can’t simply rely on Tehran’s goodwill to magically release him; there must be negotiations — give and take. Merely mentioning to Iranian officials that we would like some help in locating him — the official U.S. line for years now — won’t cut it. While Iran has an enormous opportunity to open a new chapter in its relationship with the United States and the world, it is unlikely to do so without incentives.

Shortly after the nuclear deal was reached this summer, CBS News’s Major Garrett asked Obama why my father and the other Americans weren’t included in the deal. It was a fair question, but the ensuing media storm about the exchange focused on Garrett’s phrasing and the president’s response to that, rather than on the issue itself.

Garrett explained in a follow-up interview, “In the final hours of this deal, the Iranians put other things on the table that hadn’t been previously discussed: the arms embargo on conventional weapons and ballistic missiles. If those could be introduced, it seems to me it’s reasonable to ask the commander-in-chief if other issues on the American side could have been introduced.”

Obama insisted then he was not “content” as he “celebrate[s] with American citizens languishing in Iranian jails.” Yet in October, Roll Call reported, “Democrats threw a party to celebrate formal adoption of the Iran nuclear deal,” with several White House officials attending.

There should be no celebrating. My father and four other Americans are still there, lost in the misguided euphoria over the nuclear deal. Their plights must never be forgotten, and officials have a responsibility to take immediate action to bring them home.

At the same time, Iran should know that the release of my father and the other Americans would be an opportunity to reassure potential foreign partners — both governments and private enterprises — that the country is a welcoming and safe place for their citizens and employees to work.

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. 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.