The flames briefly engulfed several cars passing near the truck, Small said, causing seven more people to be treated for minor injuries. "They were mostly just shaken up," he said.
Two surveillance cameras from nearby businesses captured the moment of the blast, said Small, who viewed the tapes.
"It looks like a very large fireball that goes completely across the street," Small said.
The four-foot-high propane tank that exploded was one of two tanks positioned on the back of the truck, Small said. The blast was so powerful, he said, that the tank landed 150 feet away in a backyard. Other debris landed in an intersection nearly a block away, he said. Officials had not determined whether the blast was accidental or suspicious, Small said.
Fire marshals were investigating with police detectives and bomb-squad officers, Small said.
Luis Rivera, 23, said his Wyoming Avenue home shook from the blast. When he saw somebody running down the street, he ran out to follow.
"I thought it was a car accident - there are usually a lot on this street," he said. "Then I came outside and saw the lunch truck in flames."
A woman who worked on the truck lay on the sidewalk, badly burned and unconscious, Rivera said. Another woman with grave burns lay on the ground nearby, calling for help, he said.
Several neighbors, he said, were able to free two children from a car damaged in the blast. The children did not seem to be seriously hurt, he said, but were among those transported to the hospital.
Police did not identify any of the injured.
The truck parks on the street daily, residents said. Lettering on the side reads "Parrillada Chapina" and some in the neighborhood said the operators are Guatemalan. Chapin is a term commonly used by Guatemalans to refer to themselves.
"It's got a big clientele," said Carlos Miranda, who works in the area. "Everybody stops there. They're good people, very friendly."
Michael Blackie was on the next corner, he said, when the explosion made him miss a step.
The fire had spread to the auto lot, next to where the truck was parked, he said.
One female appeared to have been blown from the truck, he said.
"She was on the street behind the truck. I thought she was dead. She was burned," he said. "It was horrible."
Blackie said he lived on the same block as the truck owners. They were from Guatemala and "very hardworking."
SEPTA police appeared to be the first to respond to the blast, Blackie said. They used a blanket provided by a neighbor to move one of the burned females, he said.
Small said fire trucks were on the scene within three minutes.
By 6:13 p.m., almost 45 minutes after the explosion was reported, paramedics had transported all 10 victims to area hospitals. Initial police-radio reports indicated that for a time, the city had no available ambulances to send to the scene.
A Fire Department source said the lack of available ambulances did not affect the initial response to the blast, but caused some delays in getting help to the less seriously injured.