Police face questions about London Bridge attacker

Authorities have confirmed that suspected ringleader Khuram Butt was known to them as a potential threat.
2:49 | 06/06/17

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Transcript for Police face questions about London Bridge attacker
Now to those developments in the London terror attack. Earlier this morning a moment of silence was held across the uk for the victims and we are now learning new details about the men who carried out the deadly attack. One of them was well known to authorities for his extremist views and ABC's chief investigative correspondent Brian Ross is here with the latest on that. Good morning, Brian. Reporter: Good morning, Amy. Police this morning have identified a third attacker, a Moroccan from Italy who also was well known to police as an ISIS sympathizer. Adding to questions police are facing this morning of whether they dropped the ball and failed to keep tabs on people they knew as potential threats. It was just last year that 27-year-old khuram butt was featured in a documentary aired on channel 4 in London about a group that openly campaigns for strict islamic sharia law. Here praying before the black flag of Islam. The attacker is detained by police and accuses the officers of discrimination. Because he's white and he's English, question go but all of us that were just praying, we have to stay. Reporter: Counterterror experts say the members of his group have a deep hatred for non-muslims. They see they as less than human, stabbing them, running them over like they're dogs is not so difficult, easier to imagine how someone could end up doing that. Reporter: British authorities say in 2015, they opened an investigation of khuram butt who briefly worked for the British transit service but they concluded he was not a threat. So on Saturday night as he and his two followers marched on the street to kill, authorities were no longer keeping tabs on him. This newly released security camera footage shows the dramatic moments as a restaurant manager fought off two of the knife-wielding attackers. Watch as the manager rushes people inside, barricading the door with the attackers trying to force their way inside. The manager slams the door and the attackers move on. We always ask ourselves should we have known could this have been prevented? Reporter: A second attacker has been identified as Rasheed redouane of Libyan or Moroccan descent not known to police before. This spate of attacks shows what the uk and Europe more broadly are up against with all of the individuals who are radicalized and prepared to carry out this type of attack at least inspired by ISIS. Reporter: The third attacker identified this morning, his name is Yousef zagba well known to authorities because he tried to go to Syria to join ISIS. All right, and then a lot of people are asking what about extremists who are living here in the United States on the radar known to authorities? Reporter: Well, the FBI says it has active investigations in all 50 states, but a lot fewer people who are considered active threats than in great Britain. They have 23,000 people they're trying to keep tabs on. Wow, all right, Brian, thank

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

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