Found a reference to this in a list on pro audio systems I'm a member of and thought it may be of interest here. The system discussed is called ShotSpotter and is installed in some cities (worldwide). It is intended to recognize and locate gunshots. In this case, it triggered on the sound of the crash - but did not alert authorities because it determined it was not a gunshot. In later review, the company realized there was the sound of the plane crash.

Here is a link to the news story with a portion of the audio. Be advised, it is pretty intense. The NTSB is reviewing the sounds.

Three employees of Tesla (electric car company) perished in the crash. RIP.

http://www.contracostatimes.com/news/ci_14431486