Transcript for Search for Manchester bomber's potential ISIS connections underway

Authorities here now say they know who the suicide bomber is. It turns out he was known to police. They blasted their way through his home tonight and you're about to see it here. ABC's Brian Ross. Reporter: In the frantic search today to see if others are involved, police blew open the door of the bomber's home, looking for any clues about the suspect. One officer was seen with a small book titled know your chemicals. Later, they took one man into custody nearby, identified by neighbors as the terrorist's older brother. Investigators need to focus on where day learned to build a device like this, the real question, is there a cell that goes beyond this individual? Reporter: Tonight, we are learning the bomber himself, 22-year-old Salman Abedi, was born in Manchester, the son of a family who emigrated from Libya. A college dropout, Abedi was once one of the 500 young men known to British authorities as possible terrorists. Abedi was a terrorist suspect in the uk, mi5 were aware of him, they were aware he posed a potential threat, but they didn't think he posed the imminent threat that he proved himself to do in Manchester. Reporter: Neighbors today told ABC news the Abedi family would only talk with other muslims. The neighborhood, around an area called moss side, just a few miles from the concert hall, is considered by police to be a hotbed of ISIS recruitment. Moss side is very well known. A lot of people with petty criminal pasts, involvement in gangs, getting involved instead with ISIS later on. Reporter: Manchester police have actually been planning for a terrorist attack. For some time. This training exercise was held just last year at a Manchester shopping mall, to help police plan for an attack on a crowded, soft target. Brian Ross joins us tonight. You reported there that this suspect was known to British authorities as a potential suspect. That's right. Salman Abedi was on a government watch list at some point. He wasn't being closely watched because authorities didn't believe he posed an immediate threat.

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