On Tuesday, police shared video of a person they would like to speak with in connection with the case.
"If you know this person or recognize his walk, our tip line is 484-574-1157," said police.
Police looking to speak with person seen on video in connection with teen's murder
The stabbing happened around 9:30 p.m. on April 21 on the 200 block of Bridge Street.
No suspects have been identified at this time.
Police say two people seen holding hands may have witnessed the crime and could provide clues in the search for a suspect.
Authorities say the pair were walking hand-in-hand on Bridge Street, passing Anne Street, and headed toward Dennison Avenue, at 9:39 p.m. Friday.
Action News spoke to Breanna Pierre, who said she just happen to be walking along Bridge Street when Garr was stabbed.
"Some kid fall to the ground, a man running, and a man come out of a car to help the kid on the ground, and I'm like, 'oh my God.' I was going to go over there and see what happened, but then I saw this blood," said Pierre.
Pierre said she was too far away to be able to describe the person responsible.
"It's terrible," said Upper Darby Police Superintendent Timothy Bernhardt. "How do you get to a point where kids this age can commit some type of act of violence like this? There's no words to explain it. Our hearts go out to the family, we ask everyone to keep the family in their thoughts and prayers."
The Delaware County Fraternal Order of Police Lodge 27 is offering a $10,000 reward for a tip that leads to an arrest.
Garr, of Drexel Hill, was a 10th grader at the Upper Darby School District.
Anyone with information is asked to call Upper Darby police detectives at 484-574-1157.