It all started when police say the suspect got off a SEPTA bus at 64th Street and Lindbergh Boulevard around 10 p.m.
Police say the suspect stole a bike from the front of the bus, then rode up to another 19-year-old man and shot him in the throat at 66th Street and Dicks Avenue.
The victim was taken to Penn-Presbyterian Medical Center, where he later died.
The suspect ditched the bike, but then stole an electric bike from someone else and took off. Investigators tracked the suspect to a Wawa at 84th and Bartram Avenue, where they arrested him and recovered a gun.
The victim has been identified by family as Russell Williams.
Still coming to grips with the loss of their only son, Russell and Katrina Williams say they're still waiting for Russell Jr, or RJ, to walk through the door.
"It's still like shocking because I keep thinking he's going to come home and he's not," said Katrina.
His parents say the 19-year-old loved sports and gaming, had recently graduated high school, and that he was everyone's friend.
"You could walk to the store, somebody knows him. You could go to the park, somebody knows him," said his mother.
The suspect has not yet been charged and his identity has not been released.
The Williams family says they want justice.
"You took his life for nothing," said the victim's father.
Authorities are still working to determine a motive.
Anyone with information on the case is asked to call police at 215-686-TIPS.