The robbers pulled ahead of the smoldering remains of Norco's police department and set up an ambush, because at that point they had forgotten the difference between "bank robber" and "crazed John Woo movie villain." One officer was killed in the opening salvo, and it seemed that yet another police department was about to be overrun by four maniacs with assault rifles. Then D.J. McCarty, an off-duty cop who heard about the shootout and realized they'd long since abandoned reality and stepped into Lethal Weapon world, finally arrived with the department's sole assault rifle.
As the robbers advanced on the cops, McCarty sprayed a wall of bullets over the hood of his car, forcing the robbers to retreat. His aim was haphazard and he didn't even know how to fire the thing at first, but that didn't matter. Clearly, the robbers realized this was the part of the fight scene where the good guys are finished taking an impossible beating, and proceed to rally to victory. So the tides dutifully turned, the robbers lost their steam, and were flushed out within days. Three of the culprits were thrown in jail for life for killing one police officer, wounding eight, and temporarily replacing the airspace of Norco with hot lead.
As well as receiving a small fine for breaking a little-known city ordinance forbidding automatic arms fire during work hours.
The fourth died in one final shootout with the police, because every good action flick needs a dramatic showdown.