[Shooter 2.5]

I think terrorists would have more than one firearm. It's also possible that a cell of terrorists would have killed at the same time in two or more different areas.
I still don't know if it's two people involved. One person would brag using the tarot card but I don't see why two people would have to explain to others know what they're doing. The card did say "I" instead of "We".

[Woodman]

I would not rule out a group just because of an individualistic act such as the Tarot card. There may well be some lose cannons in a theoretical group/cell who may want to “stand out” so to speak. It may also explain why they would not want this detail released to the press. Slipping up like this could cost someone their life if this was a group act.