This sounds like the scenario in Tom Clancy's last book. A bunch of rag terrorists come in thru Mexico and shoot up several malls at the same time.
In the book, the bad guys do a lot of damage but end up getting drilled by civilians and off-duty cops with CCW's. That's not likely to happen in real-life LA.
What was that book? I've been trying to remember, but cannot. It seemed the beginning of a new series.