Transcript for Did Manchester bomber Salman Abedi act alone?
but first to our chief investigative correspondent Brian Ross for breaking new details about the bomber. Reporter: Senior British officials this morning are saying Salman Abedi likely did not act alone. With French officials adding the bomber had proven lichlnks to ISIS. London newspapers published photos of the man they say was the bomber, 22-year-old Salman Abedi, born in Britain to parents from Libya. In the frantic search to see if others were involved with him, police blew open the door of his Manchester home looking for evidence of any kind including his phone and computer, among the items removed a book entitled "Know your chemicals." Authorities also want to know if Abedi used videos posted by ISIS this year with instructions on how to build a lethal suicide bomb. There is an active ISIS presence in Libya and U.S. Authorities believe Abedi may have traveled there two years ago at a time he dropped out of college. At one point Abedi was on the radar of British intelligence as a possible terror threat. One of thousands of young men under suspicion. Every day mi5 is having to make a call on who are the most dangerous, who they fear will be operational terrorists looking to carry out attacks. And, of course, they're not going to get it right every time. Reporter: The Manchester neighborhood where Abedi lived around an area called moss side just a few miles from the concert arena is considered by police to be a hotbed of ISIS recruitment. Moss side is very well known. A lot with criminal petty pasts and involved in gangs getting involved with ISIS instead later on. Reporter: Manchester police have been planning for a terrorist attack. This training exercise was held just last year in a Manchester shopping mall to help police prepare their response to a suicide bombing on a crowded soft target. Key details could come today about the nature of the plot based on what analysts discover about the bomb and how it was built. Did the terrorist mean to blow himself up? Or did it happen accidentally? Also unclear how many died from the palace itself or were killed in the panic caused by the terrorist bomb, George. Brian, thanks very much. Terry Moran is on the scene in
This transcript has been automatically generated and may not be 100% accurate.