Did they? I'd only heard they were considering the card name a person of interest, which would make sense if they'd either accounted for everyone else at the scene but the name on the card, or are looking for witnesses who might have more info.

Edit: Breaking news is that the card is indeed believed to be from the attacker. Aside from photos on social media, the suspect also left behind a bag of other weapons and a key to a U-Haul van. I'm guessing the card may have been in said bag. I'm not really sure there is much to suspect parallel construction here.