ISIS, the extremist group currently in control of vast swathes of Iraq and Syria, has been ruthlessly adept at recruiting followers via social media, encouraging them to either join the group in the Middle East or commit acts of terror at home. The problem is, despite its Whac-a-Mole–style social-media tactics, everyone can see what ISIS is doing on a public network—and according to the director of the F.B.I., they can too.

In an interview with the Huffington Post, F.B.I. director James Comey said that while Twitter was an “extraordinarily effective way to sell shoes, or vacations, or terrorism”—and, more colorfully, “the devil on your shoulder all day long, saying, ‘Kill, kill, kill’”—he also found it to be extremely good at routing out potential terrorists.

While he made it clear that he wasn’t going after everyone who ever re-tweeted something from a pro-ISIS account (i.e., academics and commentators sharing tweets for research purposes), Comey said that Twitter users who post ISIS stuff on their timelines were essentially sending a red flag to the F.B.I., which would then have to prove “beyond a reasonable doubt” that the users were actually engaging in illegal activity—i.e., plotting attacks on behalf of the group, sending money or supplies to ISIS, or actively planning to leave the country to join the group. “We're required to prove you knew what you were doing, you knew it was wrong, and you did it anyway,” he explained.

According to the Huffington Post, the Twitter dragnet can actually work:

Among the recent terrorism cases that pointed to Twitter, the feds brought criminal charges against Ali Shukri Amin, a 17-year-old from Virginia who operated the Twitter account @AmreekiWitness, simply for sending certain tweets. The government -- which in press releases alternatively referred to Amin as a “Manassas Man” and a “Virginia Teen” -- focused on Amin's tweets about ways to use Bitcoin to financially support the Islamic State, also known as ISIL and ISIS. Amin has pleaded guilty and, in a statement of facts, agreed that he had operated the Twitter account, which “boasted over 4,000 followers,” as a “pro-ISIL platform during the course of over 7,000 ‘tweets.’”…