Every mass shooting people initially think there are more shooters than there are. Even the professional cops/military often experience this problem. If you listen to the scanner recording of this event (i did) there are plenty of false reports of shootings at tropicana, luxor, during the event etc.. due to echoes.
Precisely ... live in western Colorado next to an escarpment. Jets taking off/landing at nearby airport cause a echo type effect off the walls of this escarpment ... making it difficult to determine the precise location of the aircraft. We recently had an air show featuring the Navy Thunderbirds, you could hear them but it didn't always correlate with their actual location.