Transcript for Third terror attack on United Kingdom since March

Good morning. It has happened again. For the third time in three months, terror has struck at America's closest ally, great Britain. This time, on a cool Saturday night in London. An attack crude, deadly, impossible to stop. Attackers mowed down walkers on the London bridge. Then, wearing fake explosive vests, they set out on a stabbing spree killing at least seven, injuring 48. The killers have not been identified. This come tong heels of the isis-inspired suicide bombing in Manchester less than two weeks ago. Just ahead of the memorial concert taking place later today. Theresa may, defiant. But it is time to say enough is enough. Everybody needs to go about their lives as they normally would. As a country, our response must be as it has always been, when we have been confronted by violence. We must come together. We must pull together. And united, we will take on and defeat our enemies. Strong word there is from the prime minister. Terry Moran on the scene in land. Good morning, Terry. We heard the prime minister saying life must go on. She's also saying things are going to have to change. Reporter: She is, George. That's absolutely right. Even while we're seeing forensic evidence technicians and police activities behind us, still about a block away fwr the scene of the attack, the prime minister is trying to address the shock, sorrow and increasing frustration and even anger. Perhaps the most striking thing she said, things must change. She's talking about the law. Enhanced resources for security services. To monitor the thousands of people who have fallen into the category of expected extremism. Enhanced penalties. Sentences. For terrorism offenses andlesser offenses. International agreements to regulate cyberspace. To reduce what she calls the safe spaces for ideology. People can only handle so much. They want action. No claim of responsibility. London police have arrested 12 this morning? Reporter: That's right. In the suburb of barking. A raid and 12 arrests. That shows that they have -- it seems, identified the attackers and arrested people they think are connected with this attack. We saw the same thing in Manchester 12 day ago. After the attack on the Ariana grande concert. A series of Ares. Almost as if the police, for all of the effort, and it's a tremendous effort that the British government puts to keeping people safe here. They're missing cells of people ready and willing to attack. One more sign of that defiance you talked about. That concert, Ariana grande concert, going forward tonight. Reporter: That's right. It can't be underestimated. The British people, londoners, in particular they have a hair teenage and history here of dealing with violence, going bahhing to the I.R.A. And world War II. It's the stiff upper lip. It's not a myth. There's an effort here not to freak out. To keep on and carry on. That was a saying in World War II. London can take it. You see people going about their business here. They're determined to get on top of this problem, not let it change what is one of the world's great cities. Terry Moran, thank you very much. President trump tweeting about the attacks. One of the first ones, we need to be smart, vigilant, and tough. We need the courts to give us back our rights. We need the travel ban as an extra level of safety. Do you notice we are not having a gun debate right now? That's because they used a truck and knives. The mayor of London says there is no reason to be alarmed. We're joined by Susan rice who

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