LONDON — After three years of attacks in Europe in which security services frequently struggled to explain how the perpetrators were able to avoid detection, Britain is wrestling with a different question in the wake of Saturday’s attack: How could three extremists strike in the heart of London when at least two of them had been known to authorities?

One of the attackers, Khuram Shazad Butt, was certainly not hiding his radical views.

In fact, he had revealed them to millions of TV viewers in the 2016 documentary “The Jihadis Next Door.”

What should have raised even more alarm, according to experts, was the connection between Butt and possibly another of the attackers to a well-known radical British group called al-Muhajiroun. Counterterrorism analysts had called for a crackdown on the group and its offshoots for years. Now they wonder whether a tougher approach could have saved lives and whether there are lessons for other countries with locally entrenched extremist groups.