By Walt Hunter

PHILADELPHIA (CBS) — Two sisters. Two separate hit and runs. And a lengthy police response has prompted an investigation by the Philadelphia Police Department.

An exclusive CBS 3 I-Team investigation has revealed it took police nearly five hours before the first car was dispatched to a hit-and-run accident in Northeast Philadelphia Saturday night that left 15-year-old Kylie Givens critically injured.

Police Commissioner Charles Ramsey tells CBS 3 the department has started an investigation to determine why it took so long.

“It took awhile for police to arrive. What’s important about that is the fact that there’s a crime scene and you a got a hit and run with someone injured,” Ramsey said.

Records show the first “911″ calls for the hit-and-run were received at 8:32 p.m. Medics were dispatched immediately. But the first police car from the 15th District wasn’t sent until 12:53 a.m.

Ramsey expressed concerns that the delayed response prevented a crime scene from being established and, after the long delay, valuable evidence and eyewitnesses that could help catch the suspect may have been lost forever.

Police say 72 hours before Kylie was run down, her 10-year-old sister Sarah was struck by another hit-and-run driver, suffering a severe leg injury. Police say the incidents, in different parts of the city, are unrelated.

“These are two innocent girls. It happened, say you’re sorry. Show them your remorse,” the victim’s mother Nina Doxzon-Givens said.

So far, both hit-and-run drivers remain on the loose.