Authorities are trying to determine what motivated a 30-year-old man accused of ambushing a uniformed sheriff's deputy and firing multiple shots in what they believe was a targeted killing.

Shannon J. Miles is charged with capital murder in the fatal shooting Friday night of Matthew Goforth, a 10-year veteran of the Harris County Sheriff's Office.

Harris County Sheriff Ron Hickman says there is no evidence so far that Goforth knew Miles and the attack was "clearly unprovoked."

Hickman says their assumption is that Goforth was targeted because he was in law enforcement, although they have not learned any details from the suspect that would shed light on his motive.