MARTINEZ -- A Superior Court judge on Tuesday upheld a sentence of life in prison without possibility of parole against an Antioch man convicted of first-degree murder in 2011, Contra Costa deputy district attorney Mary Knox said.

Kayl Gerard McCutchen, 22, was convicted of robbing and shooting Matthew Butler in 2009, a crime Judge Theresa Canepa referred to at Tuesday's hearing as a "merciless execution," according to Knox. Because McCutchen was 17 and a juvenile at the time of the killing, and per a U.S. Supreme Court decision, the court was required to state for the record circumstances justifying a sentence of life without possibility of parole, Knox said.

After hearing from Knox regarding McCutchen's "major participation" in the crime and his extensive criminal record as a juvenile, Canepa found no mitigating factors regarding the sentence, the prosecutor said.

Knox said approximately 30 family members and friends of Matthew Butler attended Tuesday's hearing.

Investigators said McCutchen lured Butler into a drug deal on June 28, 2009. At trial, a friend of McCutchen's who was with him the night of the shooting testified that McCutchen shot Butler while the two were in Butler's car.

The friend testified that McCutchen then ran from the car with a bag containing a small amount of marijuana. McCuthchen's attorney claimed the gun discharged during a struggle between the two men.

Contact Gary Peterson at 925-952-5053. Follow him at Twitter.com/garyscribe.