If you had that many full mags, along with IEDs, why would you just casually surrender, and not shoot it out with the cops? Additionally, 7 minutes response time in a city? Really?
For an example of what damage 3 guys, well motivated, can do to responding police forces, look at the video from Cali about 15 or 20 years ago.
Three guys using, apparently, AR-15, with 30 round mags, and they can only take out 57 people killed/wounded?
No attempt to flee and get another target? Dropping a working rifle for no reason? Mass shooting just happens to occur in a city with two major earthquakes in recent history?
This is definitely fishy, even more so with extra reasons I haven't even included.