I wouldn't worry about this too much.
If these guys were planning anything, I doubt they would take the trouble to warn us. If they're up to something, it probably has nothing to do with a mall. Most of the losses due to terrorism are due to our own security measures, political, economic and so on.
This kind of threat is designed to scare us, and get us to spend a fortune on security, to get us to violate our own constitutional principles, or maybe to provoke some amateur Jihadist out there into trying something.
"Its the shot you don't hear that kills you."