Transcript for UK raises threat level to critical after Manchester attack

Brian, thanks very much. Terry Moran is on the scene in Manchester and, Terry, we're seeing those scenes there of how the attack unfolded. You have new details. Reporter: That's right, George. With Britain's terror alert now at its highest level in more than a decade that means up to 5,000 military troops will be deployed at key sites and on streets across the country including at Buckingham palace where today they canceled the changing of the guard. That iconic London tourist event while police continue to unravel the plot three new arrests directly linked to the attack in Manchester. This morning the uk's terror level is at its highest, critical. Their assetment is not only that an attack remains highly likely but that a further attack may be imminent. Reporter: As they search for possible accomplices of the identified killer. Across Britain up to 5,000 armed troops could be deployed on the country's streets helping police keep batch in public places and at major sites like Buckingham palace. People are likely to see some military on the streets. It's all about making sure that we keep people safe. ??? Reporter: And we are learning more this morning about what happened in those moments after Ariana grande left the stage. Oh, my god. What's going on? I think people -- I couldn't find my mum. Reporter: They believe the killer detonated his bomb here connecting the arena and the nearby train station. Tuesday moments of silence across the world and in the aftermath of this attack so many are relieved. In the back of your mind you're seeing the worst. Reporter: Then you saw her. Huge relief. Huge relief. Almost like she had been born again. Reporter: And there are still many parents and families hoping against hope to see their still missing loved ones as authorities slowly confirm the identities of the dead. Emotions are running very high here. There is an election here in two weeks and voters are demanding answers. Robin. I bet they are. Okay, Terry, as we've been

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