UK threat level raised to critical after terror attack

One arrest has been made but investigators are still hunting for more suspects in the London subway bombing.
5:01 | 09/16/17

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Transcript for UK threat level raised to critical after terror attack
threat level will remain at critical meaning they believe that another attack might be imminent. The crude bomb used here was contained in a bucket, wires hanging out of it. It partially exploded during the morning rush hour. The blast set off a stampede of terrified commuters. ISIS has claimed responsibility. They say more bombs are out there. Questions loom this morning including whether there are other terrorists on the loose right now. We begin our coverage with ABC's Ian Pannell who is in London for us this morning. Ian, good morning to you. Reporter: Dan, Paula, good morning to you. From parsons green station, this was at station attacked yesterday. Police on patrol this morning. Things remain tense here and people are being vigilant so, yes, a major news. An 18-year-old man picked up the city of Dover. Why is Dover? If you're trying to flee the country Dover is one of those places you go to. You get on way ferry and go to France and you'll be in continental Europe. Two other things, one is that this investigation is continuing and that the terror threats remains critical. This morning as London recovers, an 18-year-old arrested in Dover. But the investigation continues and there's a heavy police presence at busy London train stations. Uk terror threat raised to critical. The highest possible tier meaning another attack could be imminent. Counterterrorism police and mi5 hunting for other suspects. The explosion detonating at 8:20 A.M. Yesterday morning but didn't go off as the attackers planned. Within three seconds of putting the bag down the doors closing, the explosion occurred. I turned left and I saw the fireball to my side. Reporter: People trying to escape. Women underneath me. A lady saying I'm pregnant. A little boy, HP face got smacked. Layers and layers of people screaming. People were getting crushed and we were just trampling each other and it was just every man for themselves. Reporter: Emergency responders tending to the injured after passengers escaped out the back of the train. I saw one woman being wheeled out on a stretcher her legs batb badly burned. Reporter: Many could have died in the bomb fully exploded. It was the same material used in the Manchester attack, tat. Seems the bombers got the mixture wrong and why it didn't go off as it had been planned. We're strengthening it around the country. Reporter: ISIS say they plan planted it. More than that have been foiled by the police. Well, what that means Britain is facing its most sustained terror attack incident since the 1970s in the I.R.A. Bombing campaign. This isn't a country, this isn't a city that scares easily and as you can see this is a Normal Saturday morning, a little market. There are people out with their dogs and children. Even so, people are remaining vigilant and cautious and they're on high alert. Dan, Paula. They are but Britain is a country on the lookout. Conor McGregor Cohen was a top intelligence and counterterrorism official in the department of homeland security. Good morning to you, thanks for joining us John. First and foremost, as Ian was just reporting this 18-year-old was arrested. How significant a development is this arrest and how does it change the investigation? Well, it's a significant event. It will help lead authorities to determining whether there are additional attackers who are out there. That's their number one priority at this point. They want to find out whether this person worked alone or whether there are other people out there who may be preparing to carry out an attack. Secondly they'll want to figure out was this somebody who was recruited and trained by ISIS or Al Qaeda or somebody who was a troubled youth who was inspired by what they saw on the internet. As Ian Pannell reported, Friday's attack may have involved the same type of explosives used in the bombing of that Ariana grande concert in Manchester several months ago. What do you make of that connection? Is that important? Well, it could be very important. I mean, what law enforcement authorities in Europe and London in particular want to determine whether they have a bombmaker out there working with multiple groups of people to set these bombs and try to cause these attacks to take place. So that will be one of the things they'll look at. The good news here, much of the bomb still exists so they're going to be able to get a lot of evidence from that device which will help them lead to the bombmaker. John Cohen, thank you very much.

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

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