This piece of evidence could be crucial in solving the fatal shooting of Billy Schmidt in South Philadelphia

Philadelphia police are continuing to search for the gunman who shot and killed a Penn State student near his home in South Philly over the weekend. 

Police and prosecutors are hoping a cellphone they recovered will have DNA on it that leads them to the person who shot and killed 22-year-old Billy Schmidt.

Flowers and photos surround a fire hydrant at a growing memorial for Schmidt in South Philly. The Penn State communications major was shot and killed near 20th and Durfor streets Saturday, not even a block from his home.

Schmidt's father is searching for reasons why it happened. 

"I was shocked," said Billy Schmidt, who is the victim's father. "I ran into his room. I didn't believe it."

Investigators said Schmidt left a neighborhood bar when two people approached him. Police said one of the two stole his phone.

Surveillance video shows Schmidt chasing after them, asking for his phone back. Investigators said the shooter then fired once, hitting him in the chest and killing him.

"He was a great kid, he was like my best friend," his father said. "Never had a problem with anyone, very respectful."

Schmidt's father later located the phone under a car.

Prosecutors said it will be valuable in trying to identify those responsible as detectives continue looking for evidence.

"They are working diligently and have been back out to the scene to survey and recover anything else at this time," Philadelphia Assistant District Attorney Bill Fritze said.

While District Attorney Larry Krasner was talking about an increase in his office's budget to make Philadelphia safer Monday, CBS News Philadelphia asked about the tragic circumstances of what happened to Schmidt.

"It resonates with me, too," Krasner said. "Crime is going to continue in human existence. The real question is, how do you manage it? Are you managing crime, reducing it? Reducing victimizations?"

Schmidt's grieving father wants answers.

"I want them to pay," he said. "I don't want them to get away with this."

Police also recovered a single spent shell casing at the scene of the shooting. They're also looking for more surveillance video.

Sources told CBS News Philadelphia investigators are looking to rule out if the shooting is an isolated incident, or if there have been phone robberies recently in South Philadelphia or Center City.