Who knows? Maybe not. But it is reasonable to suspect this guy is an Islamic convert.
ISIS claims him as a convert and takes responsibility for the massacre. ISIS generally does not randomly and falsely claim complicity in attacks like these.
He may not be but I think in the light of 30 years of the same pattern of Islamic jihadist attacks, its is reasonable to presume he is until proven otherwise
That hasn't been my experience. They seem to claim every terrorist attack that happens as their ow.