Police say the woman was behind the wheel of a truck and driving at a tremendous speed when she struck a small SUV occupied by a grandmother, a mother and her two daughters, ages 3 and 7.
"Just a boom, nothing we've ever heard before, it was the loudest noise," said Patty Paslay.
Anne Marie Muldoon, a chiropractor who was treating patients, ran out of her office to render aid and saw two people performing CPR on the 3-year-old.
"He was doing perfect chest compressions and another woman started doing inhalation and I had her feet and said, 'Come on baby, come on baby wake up.' But she didn't wake up," said a tearful Muldoon.
The child was rushed to St. Christopher's Hospital for Children where she was pronounced dead.
Andrew Fisher said it all began a few blocks away, where the woman struck a different car occupied by two people and fled the scene.
Fisher followed her and called police as she struck the small SUV and landed on a parked vehicle.
Fisher said she appeared to awaken from some kind of stupor.
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"She was like, 'Uh, who stole my car?' Like what do you mean who stole your car? Then the police knew she was on drugs," Fisher said.
"She seemed to be intoxicated by some means," Philadelphia police Capt. Johnny Walker said.
Police said the woman was taken to Nazareth Hospital for treatment and for blood tests. Her distraught boyfriend was seen crying near the crash scene.
"Inside of her car there was also a gun in the center compartment, so she'll obviously be charged with that. She doesn't have a permit," Walker said.
The mother, grandmother and 7-year-old sister were being treated for non-life threatening injuries. The two people in the car in the first crash are being treated for minor injuries.