It was a spur-of-the-moment shooting, a spontaneous incident over a petty gripe that left a 13-year-old boy dead from a bullet to the head.

Now, Christopher Southerland, 18, of Nicetown, faces up to 20 years in prison for participating in the fatal assault against Khisser Davis-Papther. Southerland pleaded guilty Tuesday to one count of conspiracy to commit aggravated assault, forgoing the potential of being tried for murder and related charges.

Southerland was not accused of pulling the trigger during the encounter March 10 inside the Gold Fish Chinese takeout in Nicetown. Assistant District Attorney Ed Cameron told Common Pleas Court Judge Kathryn Streeter Lewis that video showed Southerland standing in the doorway of the restaurant while a friend, wearing a mask, shot Davis-Papther in the head.

Prosecutors have charged Tymear Johnson, 19, with being the gunman, saying he fired at Davis-Papther after the younger teen and his friends made some kind of gesture toward the older teens when they got off a SEPTA bus outside the restaurant.

James Clark, captain of the Police Department's Homicide Unit at the time of the crime, described it in April as "about as senseless as it gets."

Johnson is imprisoned while awaiting trial. Southerland, who said little during Tuesday's hearing, will be sentenced at a later date, likely after Johnson's fate is determined.

Davis-Papther's family sat quietly during the hearing and declined to comment afterward.