Transcript for London Bridge attackers may have been waiting to strike for months, officials say

More now on the unfolding investigation and the apparent ISIS claim of responsibility. A series of sweeps in east London leading to a dozen arrests. And new images of the terrorists wearing decoy suicide vests. ABC news is learning the plot may have been in the works for months. Here's Brian Ross. Reporter: Police tonight say they already know a lot about the plot and the plotters. They moved quickly today into this working class east London neighborhood, making at least 12 arrests. Work is ongoing to understand more about them. About their connections, and about whether they were assisted or supported by anyone else. Reporter: Officials tell ABC news that the three terrorists have been waiting for three months. And the trigger may have been this message posted by ISIS just yesterday, calling on followers to use vehicles, guns and knives to attack. Many of those arrested today were connected to the dead terror terrorist here, seen wearing a fake bomb vest, intended to guarantee a police response that would lead to martyr dom. Reporter: Neighbors today told ABC news the dead terrorist is of Pakistani descent, the father of a toddler, with a wife who is expecting. Others said he was known for radical views, complaining his local mosque was not devout enough. He said it was not a good mosque, it was not following the really Muslim -- religion, Muslim categories. Reporter: The bbc tonight broadcast an interview with a neighbor who said he was so concerned about the man he called authorities. I did my bit, I know a lot of people did, but the authorities did not do their bit. Reporter: With more than 3,000 potential terrorists on British watch lists, authorities say they simply cannot track every suspect or stop every attack. There have been 13 or 14 plots that have been foiled in the last 12 months. And we've probably got to be prepared for more. Brian, now that ISIS has claimed responsibility, is there a fear of more attacks on the way? Yes, authorities in Britain see a wave of terror coming their way. ISIS has called on their followers to carry out as many attacks as possible during the month of Ramadan. Brian, thank you. And as we reported earlier, stories of heroism emerging. Bystanders, confronting the attackers.

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