HOUSTON — A man charged with murder in the ambush of a suburban Houston sheriff’s deputy had a history of mental illness and was once declared mentally incompetent, according to authorities and his former attorney.

Shannon J. Miles, 30, was being held without bond after an initial court hearing Monday. Prosecutors accuse him of opening fire from behind on Harris County Sheriff’s Deputy Darren Goforth on Friday night in what the sheriff called a “cold-blooded assassination.”

Goforth was shot 15 times, Harris County District Attorney Devon Anderson said in court.

Anderson would not comment on a motive, saying investigators were still trying to figure that out. When asked if it might be connected to heightened tensions around the country between law enforcement and civilians, Anderson said: “I have no idea whether it does or not.”

This weekend, Sheriff Ron Hickman said the attack was “clearly unprovoked,” that authorities believe the 47-year-old deputy was targeted because he was in uniform and that there is no evidence Goforth knew Miles.

Anthony Osso, one of Miles’ two court-appointed attorneys, told The Associated Press that his client intends to plead not guilty.