As 13-year-old Khiseer Davis-Prather sat with three friends inside the Gold Fish Chinese takeout in Nicetown, two older teens walked in, one of whom pulled out a .45-caliber handgun and shot him, a city prosecutor told a jury Tuesday.

Moments earlier on that March 2017 night, the gunman had been on a SEPTA bus stopped at a red light just outside the restaurant, Assistant District Attorney Courtney Malloy said.

One of the four boys in the restaurant allegedly had gestured toward the bus, possibly a “flip of the middle finger,” she said. That bothered Tymear Johnson, then 19, Malloy said in her opening statement at Johnson’s murder trial.

“Let’s go [expletive] them up,” Johnson said, his former friend Christopher Southerland testified. Southerland, who also had been on the bus, said he did not see what prompted Johnson to look at the boys in the restaurant. But when Johnson wanted to confront them, Southerland agreed to go along.

On their way to the restaurant, Johnson “put a mask on his face,” Southerland said. Inside, Johnson pulled out a gun and said, “What was all that [expletive] you talking about?” Southerland recalled.

Johnson, then 19, shot Davis-Prather, who was closest to him, Southerland said.

“I thought Tymear was going to use the gun as a scare tactic,” Southerland testified, saying he didn’t realize Johnson was going to shoot anyone. Southerland, who was then 17, said he had never seen Davis-Prather or any of his three friends before.

Davis-Prather died nine days later at St. Christopher’s Hospital for Children.

Southerland, who initially had been charged with murder in Davis-Prather’s death, pleaded guilty in November 2017 to conspiracy to commit aggravated assault and agreed to cooperate with prosecutors. Under an agreement, he could receive a sentence as low as three to six years in prison or as high as 10 to 20 years.

Southerland, 20, who is in custody, and Johnson, 22, stared at each other in the courtroom. At times during his testimony, Southerland appeared to smirk at Johnson, who in March 2017 had been dating Southerland’s older sister and was living with them in Nicetown.

Johnson’s attorney, Gina Amoriello, told jurors in her opening statement Tuesday that Southerland will be the only prosecution witness to identify her client as the shooter. In her questioning, she tried to cast doubt that it was Johnson who shot Davis-Prather.

Jurors on Tuesday saw video from inside the establishment that showed a masked man, whom authorities say was Johnson, pull out a gun, walk toward the four boys at a table, raise his arm, and fire at the right side of Davis-Prather’s head. As the video played, two jurors stared with their mouths agape at the sight of the abrupt shooting and seeing Davis-Prather fall to the ground.

The trial continues Wednesday before Common Pleas Court Judge Rose Marie DeFino-Nastasi.