The Missouri Supreme Court has affirmed the conviction of a southeast Missouri man who was sentenced to death for the ambush killing of a state trooper in 2005.

Lance Shockley of the southeast Missouri town of Van Buren was convicted of first-degree murder for killing Missouri State Highway Patrol Sgt. Carl Dewayne Graham Jr., who was shot in the back of the head just as he had returned home from work. Graham had been investigating Shockley for a fatal traffic accident.

Shockley has been on death row since 2009. His appeal cited a litany of alleged procedural errors at trial, claimed a juror was tainted and said the sentence didn’t fit the crime. The Supreme Court disagreed in a unanimous ruling.

Attorney General Chris Koster issued the following statement on the ruling:

“The entire law enforcement family to which we belong was devastated by the brutal and senseless murder of Sergeant Dewayne Graham. While the loss suffered by Sgt. Graham’s family endures despite the ruling, I hope that today’s judgment by the Court can bring them a small measure of closure. My prayers continue for Sgt. Graham’s family, as well as for all law enforcement officers who put their lives on the line every day.”

(The Associated Press contributed to this report)