LONDON -- Thirty-year-old Rachid Redouane, one of the London attackers, had claimed to be Moroccan and Libyan. He was not known to police.

But police were aware of the other attacker they identified Monday: 27-year-old Khuram Shazad Butt, a naturalized British citizen born in Pakistan.

He was even seen in a 2016 documentary that focused on homegrown jihadism, praying with an ISIS-style flag.

Butt seen in a documentary "The Jihadis Next Door" Channel 4

On Sunday, police raided an apartment complex on London's east side, arresting a dozen people. Some residents told CBS News they recognized Butt; They knew him by the nickname Abs.

Neighbor Erica Gasparri said she reported him to police for recruiting children.

"He said to me, 'Because the kids need to be saved.' Saved from what? 'From this world. This world of war,'" she said.

Ken Chigbo CBS News

Neighbor Ken Chigbo said Abs approached him on Saturday, asking about a rented moving van he was using.

"He started asking questions, how much, this that and the other. It wasn't until now that it started sinking in really," Chigbo said.

Michael Mimbo said that after the attack he realized the possible significance of the white van he'd seen speeding away from their apartment complex on Saturday.

"Someone that, you know, you commonly said hello to," Mimbo said. "It was really hard to believe, but then all the evidence (is) there."

The 12 people arrested at the apartment complex have been released. But Monday night, British media reported that a dozen Molotov cocktails were found in the back of the attackers' van. Police are still trying to figure out if the three men in the attack had help from anyone else.