A prosecutor told a Philadelphia jury Monday that 14-year-old Duval "DJ" DeShields was "executed" in 2015 by Dimitrius Brown because DeShields brandished a BB pistol at Brown's younger brother while breaking up a North Philadelphia street fight.

"This case is not a whodunit; it never has been, it never will be," Assistant District Attorney Allison Ruth told a Common Pleas Court jury of 10 women and two men in her opening statement. "It was this defendant."

Ruth said she would ask the jury to find Brown guilty of first-degree murder, which would carry a mandatory sentence of life in prison without parole.

Defense attorney Benjamin Cooper, however, promised the jury that Brown, 20, had an alibi witness and was not present when DeShields was shot once in the head during a gathering at 8:30 p.m. Oct. 12, 2015, on 10th Street near Thompson Street.

"This is a case about the streets: the streets of Philadelphia, the streets of North Philadelphia," Cooper said. "This is a case about the streets and about survival."

After two witnesses testified Monday, it appeared the streets were winning.

The first alleged eyewitness to the shooting, a 16-year-old, recanted his 2015 statement to detectives. He called it a "forgery" and denied speaking to detectives about DeShields' slaying.

In court, the boy — the Inquirer and Daily News do not identify juveniles — said he did not know or recognize Brown from that night. Under tortuous questioning by Ruth, the juvenile denied every line of the statement he purportedly gave detectives on Oct. 19 and signed in their presence.

He said he was only testifying because "I was subpoenaed to come here or I was going to be locked up."

Ruth asked about a statement by the boy's mother that he had to leave Benjamin Franklin High School after being assaulted when it became known he gave a statement. The boy denied it, saying, "It was bad grades."

DeShields' death stunned his community, where he was known for his dancing and love of music, and for how he handled the 2013 death of his mother from cancer, and his older sister's death from the disease a year later.

According to testimony at Brown's preliminary hearing in December 2015, DeShields had seen a group of preteen girls and boys fighting in his North Philadelphia neighborhood earlier on Oct. 12. Friends said he brandished a BB pistol with an orange-tipped barrel and the fighting ended.

That night, DeShields and friends were riding bicycles when Brown allegedly confronted him at 10th and Thompson.

Witnesses testified that Brown grabbed DeShields by the collar and demanded: "What makes you think it's cool for you to pull a gun on one of my young bulls?"

DeShields broke free, turned, and ran, and Brown shot him once in the head, witnesses testified. DeShields, an eighth grader at Paul L. Dunbar School who had turned 14 a week earlier, died the next day.

SEPTA Police Officer Samuel Washington testified that he had rushed the mortally wounded DeShields to Hahnemann University Hospital. As he helped lift DeShields out of the SUV and onto a gurney, the BB gun fell to the ground, Washington said.