The familiar refrain we hear after nearly every terror attack in a Western country is that the “terrorists were previously known to authorities.” In the latest attack in London, however, one of the attackers was known to anyone who watched a 2016 television documentary called, “The Jihadist Next Door.”

According to ABC News, police have identified 27-year-old Kuram Shezad Butt as one of the men in the van that plowed into pedestrians at a crowded market on Saturday, killing seven. Butt was featured in the documentary as a known member of a small Islamic extremist group, Al Muhajiroun, led by Anjem Choudary and Siddartha Dhar.

Choudary, a British citizen, was convicted of supporting Islamic State in 2016 and sentenced to 5-1/2 years in jail. ISIS claimed responsibility for Saturday’s attack on its Amaq News Agency on Sunday.

This serves as a frustrating reminder that we can know exactly who people are, but until they actually do something illegal law enforcement can only watch them and wait.

However, in this case, with Choudary’s group being banned from the U.K. in 2010 and 12 people arrested for possibly being involved in the plot, one would think U.K. law enforcement would’ve been on top of Butt and his cohorts.

Banning Choudary’s group now looks like the U.K. was simply looking to feel good about itself in a “Look, we don’t allow extremist views here” kind of way. But clearly they should now know that just because you ban one thing doesn’t mean it won’t still exist, possibly even getting even stronger because it’s forbidden.

Either way, if British Channel 4 could find this guy and feature him as a person associated with a banned group, then law enforcement should have done something.