In the heart of Los Angeles' smoldering, gritty skid row sits the LAPD Central Division. Inside, one man with the weight of a city-wide epidemic on his shoulders waits for the phone to ring -- another call to investigate another killer.
Detective Felix Padilla has been working in the LAPD's traffic division for over a decade and focuses on a special breed of killers, ones who don't use a knife or a gun, but a car.
"The things that I've done, the things that I've seen, the average person doesn't see in their lifetime. I think most cops can agree with that, for better or worse," Padilla told ABC News "20/20."
Watch the full story on "20/20" on Friday, Aug. 23 at 10 p.m. ET
The epidemic, as L.A. Weekly called it in a series of articles, is hit-and-run incidents. According to Padilla, roughly half of all car accidents in the city are hit and runs, and his division alone sees an average of 8,000 incidents per year. The ones he spends his time on are the worst of the worst accidents, involving fatal and serious injuries, often times pedestrians. This year, Padilla said there have been 16 fatal hit-and-runs.
"Every case has a face, in the beginning. And, depending on the severity of it, it stays with you a long time, something that you remember, sometimes forever," Padilla said.
Hit-and-run accidents are notoriously hard to solve. For Padilla to place a suspect behind the wheel in an accident he said he needs a perfect mix of helpful eyewitnesses, forensics -- such as car parts at the scene that can be matched to a suspect's vehicle -- and DNA from the victim once a car is recovered.
Without it all, putting a car at the scene, and identifying a specific driver behind the wheel, Padilla said he has nothing. Most often the biggest impediment to his cases, he says, is finding people willing to come forward with information.
"Witnesses are paramount in an investigation like this," he said. "We can arrest the car, meaning I can impound it. But what good is that if I don't have the person who was driving, you know?"
Padilla says one of the biggest reasons for the high rate of hit-and-run incidents in L.A., and a lack of witness participation, are the high numbers of undocumented immigrants living in the city, people driving without a license and afraid to interact with police. According to a recent report by the University of Southern California's Center for the Study of Immigrant Integration, an estimated 1 in 10 residents living in Los Angeles County is undocumented.
"I was involved in the accident, I was afraid once I realized I didn't have a driver's license,' that phrase is what I get on a lot of these hit and run interviews, 'I was afraid because I didn't have a driver's license,'" Padilla said.
That leaves nearly half of his cases, saved in binders throughout his office, with a white "unsolved" sticker. It's especially hard for Padilla when the case is a fatal hit and run.
"It's not so much that, you know, I'm putting something away. It's just that I've left a family without any answers. It's sad and just the worst part about my job," he said.
With a fatal hit-and-run, it is a murder, plain and simple, and Padilla said that the driver will have to carry with them even if they escape to drive another day.
"They've got to live with this, for the rest of their life," he said. "Some people man up, and they have turned themselves in. Those cases are very far and few."
So if the drivers won't do the right thing, stop and hold themselves accountable for an accident or render aid to a person they hit, Padilla said, it is the citizens that need to step up and help the police get control of the roads. Immigration status isn't important to him, he said, when a person can help locate a suspect, or give a broken family closure. So he waits for the next call sure to come, the next scene of the crime, hoping he will have all the pieces of the puzzle.
"I think if we let the public know that these crimes are so frequent that we need their assistance, once they grasp that concept, you know, they will be able to help us more," he said. "The more they help us, the more cases we'll solve."