Transcript for Search continues for Manchester bomber's possible ISIS connections

Jimmy: Welcome back to a special decision of "Nightline." As we're learning key details about the suspect in the attack on the Ariana grande concert in Manchester, England. The question tonight, did he act alone? Here's ABC's chief investigative correspondent Brian Ross. Reporter: The London tabloid "The sun" published the first picture tonight of the man it said was the bomber. Goo-year-old Salman Abedi, born in Britain to parents from Libya. Tuesday in the frantic search to see if others were involved with him, police blew open the door of his Manchester home, looking for evidence including his phone and computer. Among the items officers removed, a book entitled "Know your chemicals." He's looking at whether the dead terrorist is acting alone or part of a group. Reporter: Authorities want to know if Abedi used videos, posted by ISIS this year, with instructions how to build a lethal suicide bomb. I think the use of a bomb here as opposed to a car or a knife demonstrates a much higher degree of sophistication by this individual. Really suggesting that he probably did not act alone. That he certainly had some advice on how to create the bomb. Reporter: There is a huge and active ISIS presence in Libya. And U.S. Authorities believe Abedi may have recently traveled there. At one point, Abedi was on the radar of British intelligence. As a possible terror threat. One of thousands of young men under such suspicion. Abedi was a terrorist suspect in the uk. Mi5 were aware of him, aware he posed a potential threat. But they didn't think he posed the imminent threat he obviously proved himself to do in Manchester. Reporter: The Manchester neighborhood where Abedi lived, around an area called mossside a few miles from the concert arena, is considered by police to be a hotbed of ISIS acruitment. Mossside is very well known. A lot of people with petty criminal pasts, involvement in gangs, getting instead involved with ISIS later on. Reporter: A man taken into custody, identified as the terrorist's older brother. Neighbors said there was nothing they saw in Abedi's devoutly religious family that would suggest any ties to terrorism. No, of course not. There was no religion condoning, saying killing people is right. Reporter: Manchester police have actually been planning for a terrorist attack. This training exercise was held just last year at a Manchester shopping mall to help police prepare their response to a suicide bombing on a crowded soft target. We've seen over the past that ISIS has targeted soft target. Bars, restaurants, sports stadiums, now a concert. It really is a result of learning about how to create the most carnage possible. ??? Reporter: It was less than a year ago on a crowded Saturday night at the pulse nightclub in Orlando, Florida, when an isis-inspired gunman easily got past limited security and opened fire. 49 people died in the worst mass shooting in U.S. History. In Paris, ISIS orchestrated the attack on a soccer stadium. At the Bataclan concert hall, where a popular American rock group was performing. More than 130 were killed there. A perfect target for the terrorists. These are targets that represent western civilization. Which they see as lascivious. Which they see as counter to their very strict version of Islam. Reporter: Both ISIS and Al Qaeda have been posting new calls for their followers to attack large gatherings any way they can. Just two months ago, there was the attack with a driver mowing down pedestrians on the sidewalk of the parliament bridge in London. And last year the truck attack in nice, France, on bastille day. Tonight -- Oh my god. Reporter: Officials in great Britain say the Manchester attack, with a suicide bomb, showed more planning and sophistication and evil. Young teenage girls attending a concert. And they are now the target for an attack like this. Really demonstrating how ruthless ISIS is. Reporter: For "Nightline," Brian Ross, ABC news, New York. Our thanks to Brian Ross. We turn to potential national security official Richard Clark, potential FBI special agent in charge, Richard Frankel. Richard Clarke, let me start with you. If this suspect was on a watch list, why was he not actually being watched? Because there are test of thousands of people on watch lists. And all that can be done when someone is on a watch list is their electronic media use can be monitored, perhaps their phone calls can be monitored. You can't have eight agents following around 10,000, 20,000 people. And it takes eight agents or more to follow somebody around 24 hours a day. Richard Frankel, we've been talking for years about the problem of soft targets. It was horrifically displayed overnight in Manchester. What do WRE we do about this problem? Without getting into the tactics at NYPD or other police departments use, we're pretty good here in the U.S. As far as soft targets. They do things that make it harder to attack those soft targets. But as we saw in times square it still can happen. It's really not the soft target that's the problem, it's pre-soft target. The concert was a soft target, but the area outside that soft target is actually where he struck. You're saying we can never protect the area outside of a soft target? You can, but as you protect that you go further out. You go further out. And you get to the point where how far out are you going to go before you're too far away? So it's hard. Back to the suspect here, Richard Clarke. As you look at the facts coming in now, as we learn more about him, does your intuition tell you he was part of a larger cell? Or that he acted alone? I think this is a pretty sophisticated bomb. Himself. And what British intelligence and police will be doing now is looking at his cell phone use, his social media, his e-mail use, and seeing who else he was in regular contact with. You'll probably find out that there was a small cell, maybe just the brother, but maybe three or four other people. What do you make of the fact that at least the suspect was born in the uk, he grew up in that society, that he then allegedly attacked? Some of the stuff we've seen over time from these attackers are that they're not the immigrant population. They're actually the next generation. Going to the time squares bomber, the subway bomber here in New York. As we see in Manchester. He was not an immigrant, he was born in Manchester and has now joined the terrorist networks. Richard Clarke and Richard Frankel, thank you very much, really appreciate it.

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