Four Muslim men are accusing the FBI of using the "no fly" list as an threat in order to coerce them into cooperating with terror investigations. It's easy to see how it would be an effective threat: the highly secret no-fly list essentially creates a form of house arrest from which it has been nearly impossible to be removed. The men filed the complaint in federal court in New York on Tuesday, The Washington Post reports.

In the lawsuit, one of the men, Awais Sajjad, says that he went to Kennedy Airport in September 2012 but was prevented from boarding his flight to Afghanistan. FBI agents questioned him, demanding he cooperate with their investigations and reminding him that they had the authority to take him off the list.

Jameel Algibhah's story is similar, as told by The Guardian. He was asked to infiltrate a mosque in Queens, with an FBI agent allegedly reminding him that, “We’re the only ones who can take you off the list." And there's Naveed Shinwari, who had flown to Afghanistan to get married. On his return to the United States, he attempted to fly from Nebraska to Connecticut for a new job, and was told that he was not allowed to fly. "The more you help us, the more we can help you," an agent told him.

"The no-fly list is supposed to be about ensuring aviation safety, but the FBI is using it to force innocent people to become informants," Ramzi Kassem, associate professor of law at the City University of New York told the Post. "The practice borders on extortion."

To the Guardian, a "former senior FBI official" denied that the list was used to put pressure on possible informants. But it's easy to see how it could be used that way.

It appears that, of the 800,000-plus people on the list, a total of one person has successfully petitioned to have her name removed. In January, a U.S. District Judge ordered that Rahinah Ibrahim be removed from the list, according to Wired. Ibrahim was placed on the list after agents visited her apartment near Stanford University, where she was enrolled, to ask about possible links to a terror organization in Malaysia. Ibrahim was not even allowed to return to the United States to participate in the civil trial that she eventually won.

The rationale for the no-fly list is obvious, but it's also clear how it could be abused. It's a form of proactive punishment, limiting the ability of people — including American citizens — to travel internationally or to travel easily within the United States. For people like Shinwari, the effect of the no-fly list is to separate them from their families, creating a sense of desperation that could be an effective motivation to help the government in whatever way they can. The charges are still allegations, unproven. But it seems hard to believe it would never have occurred to any enterprising FBI agent that this might be a useful tactic.