One of London Bridge attackers was well known to authorities, police say

Khuram Butt was featured in a 2016 documentary about a London-based group that openly campaigned for Sharia law.
2:25 | 06/05/17

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Transcript for One of London Bridge attackers was well known to authorities, police say
As you know, new video of the attackers before the attack, a broadcast that included one of the three attackers. He's seen in a TV documentary expressing his extremist views. How did British authorities take their eyes off of him? And we learned he had a newborn. His wife delivering in recent days. Here's ABC's Brian Ross. Reporter: Authorities tell ABC news tonight that the attackers, seen in this video in the midst of their terror rampage, were led by a man known to police for years as a potential threat. We always ask ourselves should we have known and could this have been prevented. Reporter: It was just last year that butt was featured in a documentary aired on the British channel 4 about a group that openly campaigns for strict sharia law, here praying before the black flag of Islam and then getting into a confrontation with police. Of the some 3,000 people authorities considered active threats, this group is among the most extreme. The members of this network which number probably around in the hundreds are on the more dangerous end of the spectrum. Reporter: A second attacker was identified as rachid redouane, of Libyan or Moroccan descent, who the police both men come from this east end neighborhood in London, where @ police arrested at least neighbors said butt, the father of a toddler and a newborn, of Pakistani descent, held a backyard barbecue last week in his apartment's garden area that some now say seemed like a farewell gathering. Others told ABC news the police never responded when they called to report the threats butt was making against non-muslims. In the name of Allah, we believe, I'm even ready, if I have to, even to kill my own mother if she comes against Allah. Reporter: Authorities tonight say the three men had been waiting for several months to carry out their attack. And that their decision to act was triggered by this message posted by ISIS just hours earlier, urging followers to attack during Ramadan with vehicles, guns and knives. That's the question, were they responding to that message? ISIS has claimed responsibility for the attack. Brian with us now, do investigators believe they were inspired by ISIS. We know the leader of the London group is now in Syria, playing a prominent role with ISIS. All right, Brian Ross and your team will stay on it Meanwhile president trump

This transcript has been automatically generated and may not be 100% accurate.

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