A 22-year-old South End man waited outside a public housing development building for an older rival before shooting him once in the back of the head, killing him, earlier this year, a Suffolk County homicide prosecutor said at the alleged gunman’s Superior Court arraignment today.

GARRETT JACKSON (D.O.B. 7/13/87) is charged with first-degree murder, unlawful possession of a firearm, unlawful possession of ammunition, and unlawfully carrying a loaded firearm for the Feb. 11 shooting of 38-year-old Thomas Speed outside of the Lennox Street Housing development. Acting on a request by Assistant District Attorney Gretchen Lundgren, Suffolk Superior Court Clerk Magistrate Gary D. Wilson ordered Jackson held without bail pending trial, scheduled today for October 2010.

“The facts of this murder are straightforward,” Lundgren told the court. “They clearly and quite chillingly reveal that the murder of Tommy Speed was nothing short of a planned execution.”

Lundgren said the two men knew one another prior to the fatal shooting and that Jackson considered Speed a rival. At about 8:00 the night of the incident, Lundgren said, Speed was on Trotter Court within the Lennox Street housing development.

“At the same time,” she said, “the defendant was waiting calmly outside of 18 Trotter Court for the right moment, armed with a semi-automatic gun.”

When Speed entered the front door of the building, he was “completely unaware that the person he knew hanging out front was there to kill him,” she said.

“Quietly, the defendant fell in step behind Mr. Speed as he walked through the door and callously and brutally shot [Speed] at almost point-blank range in the back of the head,” Lundgren told the court.

Following the shooting, Jackson allegedly fled the scene on foot. Responding medical personnel pronounced Speed dead of his injuries.

Boston Police Department homicide detectives together with Suffolk prosecutors conducted a months-long investigation that led to Jackson’s identification as the gunman and his July 19 arrest on murder and weapons-related charges. The Suffolk County Grand Jury indicted him on Sept. 29.

Jackson is represented by attorney Barry Wilson and is expected to return to court on Oct. 22.