The jury deliberated for just over three hours Saturday before returning the verdict against 24-year-old Richard Poplawski. He was found guilty of all 28 counts he faced.
Poplawski was found guilty of first-degree murder in all three killings, the three most serious charges he faced.
The trial now enters a penalty phase in which the jury will hear evidence about his mental state, background and other factors before determining if he gets the death penalty or life in prison without parole.
Allegheny County prosecutors had said a mountain of evidence provided by 41 witnesses and 500 exhibits points conclusively at Poplawski as the gunman.
But Poplawski's attorney tried to convince the jury that reasonable doubt remains about whether Poplawski's mother, Margaret, was involved - even though she's never been accused of a crime in the April 4, 2009, incident. Prosecutors say no evidence suggests she had a role in the shootings.
