LONDON — One of the men believed to have carried out a deadly weekend attack in central London was a known radical Islamist who was filmed unfurling a black flag resembling the one used by the Islamic State group and raised the suspicion of a neighbor after allegedly trying to lure local youngsters to join his jihadist campaign.

On Monday, British police identified that man, Khuram Shazad Butt, a 27-year-old Pakistan-born Briton, as one of the assailants, saying he was known to authorities, though they had no evidence he was planning an attack. They identified a second attacker who had not aroused suspicion prior to Saturday’s rampage that killed seven people.

As details about Butt emerged, however, they prompted questions of whether he could have been stopped sooner.

He had appeared in a documentary, “The Jihadis Next Door,” that aired on British television last year. Neighbors identified Butt from the film’s footage Monday, pointing to a scene in which he is shown participating in a provocative prayer session at Regents Park, near London’s biggest mosque and helping to display a black flag covered in white Arabic lettering similar to the one used by the Islamic State group, which took responsibility for the attack.