If he no longer lived there they have NO right to go in or get a warrent IMHO.
He DOES still live there. Shame on the FBI if they don’t have a warrant, but I’d be stunned if they didn’t. Assuming they do, then open the damn door...this demon-seed nutjob just killed six people including a nine-year-old girl, and the word “alleged” doesn’t apply when there’s hundreds of witnesses.