days. here's abc's brian ross. >> reporter: authorities tell abc news tonight that the attackers, seen in this video in the midst of their terror rampage, were led by a man known to police for years as a potential threat. >> we always ask ourselves should we have known and could this have been prevented. >> reporter: it was just last year that the ringleader butt was featured in a documentary aired on the british channel 4 about a group that openly campaigns for strict sharia law, here praying before the black flag of islam, and then getting into a confrontation with police. of the some 3,000 people authorities considered active threats, this group is among the most extreme. >> the members of this network which number probably around in the hundreds are on the more dangerous end of the spectrum. >> reporter: a second attacker was identified today as rachid redouane, of libyan or moroccan descent. both men come from this east