Transcript for 8 arrested in deadly London terror attack

As we just heard, that suspect was known to authorities. There was no meet claim of responsibility but British prime minister Theresa may called the violence a sick and depraved terrorist attack. Let's go to ABC's Alex Marquardt with the latest on the investigation right now. Good morning, Alex. Reporter: Good morning, Amy. That's right. He was also a British national who was known to the intelligence services and this morning the authorities are saying that he was inspired by what they're calling international islamist terrorism. Overnight raids carried out in London, the city of Birmingham and elsewhere as police and intelligence services hunt for information on the background of the attacker and any possible ties to terror groups. The uk's top counterterrorism official announcing this morning eight were arrested in the six raids carried out. This Birmingham apartment searched by the authorities, it's still unclear what links those arrested had to the attacker. A witness would works near that apartment saying, quote, the man from London lived here. The bbc reporting that the gray Hyundai Su vuchlt that the attacker used to plow into pedestrians on Westminster bridge was rented in a nearby suburb. Little has been revealed about the identity of the attacker. Seen here after he was shot by police. Continues to go on with our investigation that this attacker acted alone inspired by international terrorism. ??? Reporter: Atack happening on the anniversary of last year's deadly ISIS bombings in Brussels. Counterterror officials say 13 plots have been thwarted in the uk in the past four years but stopping a lone wolf using such basic terror tactics can be next to impossible. It's very hard to prevent an attack if you like a low tack attack happening once an individual is inspired to carry one out. If you haven't got the intelligence to know it is happening it is very, very hard to prevent lives from being lost once the attack actually takes place. Reporter: Until now the uk has been spared the terror attacks that we've seen across Europe over the past two years. For now officials say there are no known plots in the works, but the threat level is at its second highest level meaning that sadly another attack is highly likely. Robin. Alec, thank you. We'll bring in Martha Raddatz

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