Allegheny County deputy district attorney Dan Fitzsimmons told a jury that a man killed in Duquesne two years ago at a Sweet 16 party was the intended victim.
But he did not give any explanation on Monday afternoon as to why Quentin Ingram wanted Drew Bracey dead.
Mr. Ingram is charged with homicide, seven counts of aggravated assault, reckless endangerment and a firearm count.
Reginald Pearson and his wife were hosting the party for their daughter in the basement of their home on Fifth Avenue in Duquesne, Mr. Pearson testified, when two men he did not know entered through the basement door.
A short time later, he said, one of those men started firing into the room.
Bracey, who attended East Allegheny High School, was shot seven times, and seven other young people were injured, Mr. Fitzsimmons said.
Mr. Pearson told the jury Monday afternoon that as soon as the shooting happened, he followed the men, who jumped into a passing car. He followed them for several minutes to the Rankin Bridge, he said, before losing them.
Although he didn't know Mr. Ingram, Mr. Pearson said he was able to identify the man in a photographic lineup.
"I seen him put out a semi-automatic weapon," he testified. Later, though, he added, "To tell you the truth, I didn't see who shot Drew."
Defense attorney Christy Foreman told the jurors during her opening statement to pay close attention to Mr. Pearson's testimony -- noting that his statement to police and testimony at a preliminary hearing differed.
She said Mr. Fitzsimmons' opening was "just a story."
"Listen to all the evidence -- or the lack thereof," she said. "By the time the commonwealth rests, the story it gave you just now will cease to exist."
Paula Reed Ward: firstname.lastname@example.org, 412-263-2620 or on Twitter @PaulaReedWard.