According to the NYTimes:
Alec Baldwin discharged a prop firearm on the set of a Western he was making in New Mexico on Thursday, killing the film’s director of photography and wounding the movie’s director, the authorities said.
While filming for his new movie called "Rust", a prop gun was fired by Baldwin as part of the scene -- It mysteriously released actual bullets. An investigation is underway by the Santa Fe Sheriff's department to determine how this could have happened.
Back in the early 90's, a very similar situation to this happened to Bruce Lee's son, Brandon Lee, while on set filming 'The Crow' back in 1993. This also caught a lot of public attention.
The pics below show the distressed & distraught actor on the phone with the Sherriff's office following the incident.