Jefferson was fatally shot around 5 p.m. on the 1500 block of Germantown Avenue.
Police say Jefferson was walking in the parking lot to a prayer service with another man when the gunfire erupted.
"I counted nine very loud rapid thuds one after the other. It made me hang up, go outside, and see what's going on," recalled Ori Beckerman from Kensington, who witnessed the incident.
"You can clearly see our victim walking to this mosque for a prayer service. He was walking with another male. You see the shooter run up behind the victim, and from just a few feet away, begin firing shots," said Philadelphia Police Chief Inspector Scott Small.
The mosque confirmed that Jefferson was known to pray there from time to time.
We spoke with people who were already inside the mosque when the shots rang out.
"I heard shooting from inside the mosque, and I was going outside, and I went back to ask the management to call the cops," recalled Ashraf Abdel Razek of Hatboro.
Police provide update on deadly shooting outside mosque
Surveillance video shows Jefferson collapsing in the parking lot as the gunman stood over him and kept firing execution-style, according to police.
A total of 17 shots were fired in the incident, according to Small.
Jefferson suffered a gunshot wound to the head and multiple gunshot wounds to the torso. He was rushed to an area hospital where he was pronounced dead.
Chopper 6 was overhead as officers cordoned off the parking lot, which is located near the Al-Aqsa Islamic Society along Germantown Avenue.
No arrests have been made and no weapons were recovered.
Police say no other injuries were reported.
The suspect was last seen fleeing on foot before jumping into a dark-colored sedan.
Pennsylvania Gov. Josh Shapiro responded to the tragedy on X, posting, "As an investigation into this killing proceeds, I'm praying for Philadelphia's Al-Aqsa Islamic Society community, a place I've been several times, and hoping they find peace in each other and in their faith."
Mosque leaders had no further comments on the shooting when Action News spoke with them.
Anyone with information is asked to call police at 215-686-TIPS.