The gunman who killed three Louisiana law enforcement officers left behind a manifesto that is shedding light on his motives.

FOX's Tonya J. Powers has the story:

A manifesto written by the man who ambushed and killed three Baton Rouge law enforcement officers has surfaced and gives a look at his motive behind the shootings.

Gavin Long used the alias "Cosmo", and emailed the letter to an Ohio man who says he didn't know long personally, but had gotten other emails from him after long commented on his YouTube videos this year.

In the letter, long described his actions as a "necessary evil", saying it was to create "substantial change within America's police force".

Long opened fire on the officers on Sunday morning after they'd seen him at a convenience store. Police returned fire, killing Long.

Tonya J. Powers, FOX News.

Follow Tonya J. Powers on Twitter: @TonyaJPowers