A brilliant fireball was observed over southern California, Nevada, Arizona and Sonora on Wednesday evening, September 14. Hundreds of reports have been submitted to the American Meteor Society, and even a few to this blog. The best site for learning more about this event and other fireball events is at the American Meteor Society. Their page dedicated to this fireball (including a map of all reported eyewitnesses) can be found here.
An eyewitness in southern California was able to get a video of the event.
Down in Hermosillo, Sonora, Mexico, Salvador Aguirre picked up the fireball on his all-sky camera. In the image below, the fireball is seen low on the horizon to the NNW of Hermosillo.
So what caused the fireball? Most likely it was a small asteroid, no bigger than a basketball, hitting the upper atmosphere at 20-40 km per second. Based on the behavior of similar objects, it first became visible at an altitude of ~90 km and ceased to be visible at an altitude of ~20-30 km. Though some material may have survived to impact the ground as small meteorites, at least 90% or more of the asteroid burned up in flight.