They've uncovered alleged abuse cases that the agency left unexamined for years -- even some that ended with fatal results.
Elsie Fossum died from injuries she sustained at Claremont Place, an assisted living facility in Pomona. Seven years later, her nephew, Jim Fossum, says the family is still waiting to hear from investigators at California's Department of Public Health about how his aunt died.
"It was being investigated by the department. You figure they're going to find something if there was something there, not that they'd just put the thing away and forget about it essentially," said Fossum.
The facility initially told the family that Elsie Fossum died from injuries she got when she fell down.
"That wasn't the result of a fall, that was punching," said Beverlee McPherson, former nursing director at Claremont Place.
McPherson visited Fossum in the hospital.
"Her face looked like Muhammad Ali did a dance on it. And you could see knuckles. Her eyes were so badly swollen and just hanging. She was miserable. It was very sad to see," said McPherson.
McPherson believes that the nursing assistant who worked the night shift beat the elderly patient in her care. Elsie Fossum died less than three weeks later from her injuries. Claremont Place declined to comment for this story.
The coroner's report concluded she was potentially the victim of an assault. But despite evidence pointing to a crime, the Department of Public Health's investigators sat on the case.
Yearly investigations totals obtained by the Center for Investigative Reporting from the Department of Public Health show there was a steady increase in outstanding complaint cases at the time Fossum died. And that backlog was especially severe in Southern California, where she lived.
Marc Parker believes that's because some investigators weren't doing their jobs. Parker worked at the department for 24 years, and he oversaw investigations at the time of Fossum's death. He inspected the Southern California office and says what he found there was chaos.
"I was appalled. There were hundreds and hundreds and hundreds of unassigned, uninvestigated complaints in file drawers," said Parker.
Parker says his superiors at the department pressured him and his investigators to close cases quickly and efficiently. They were ordered to wrap up at least 10 cases a month.
Dr. Ron Chapman took over the Department of Public Health in 2011. He admits the department's investigations section was poorly managed, but he says now they're making changes.
"A backlog for investigating complaints is inexcusable, should not have occurred. We've made a lot of progress since then. So today, any complaints that come in, they're being screened within 48 hours and we're not building a backlog today," said Chapman.
The data obtained by our media partners at the Center for Investigative Reporting shows the Department of Public Health has reduced their backlog in record time.
But Parker says the new policy discourages investigators from visiting the scene of potential crimes. Instead, investigators do most of their work over the phone.
"If you don't go eyes on, interview people directly, talk to people directly, you're going to miss huge amounts of information, and that places the public at risk," said Parker.
The data shows that even though investigators are closing more abuse cases, they're rarely taking action. The department is revoking far fewer caregivers' licenses, and they're sending fewer cases to the Department of Justice to the attorney general's office for prosecution.
The department defends its policy. But Chapman admits he's troubled that fewer cases are being referred for legal action.
"We don't understand that decline in numbers. It's very concerning to me, and we're looking into it," said Chapman.
That's little comfort to Fossum's family. Despite questions surrounding her death, her case was closed in 2012 without any further action.
In an email, the department admits it should have "identified this allegation as a high priority," and says, "We regret that did not happen."
"You know as we baby boomers get older, there's more and more people that are going to be living in these types of facilities and you've got to have decent agencies that monitor these situations," said Jim Fossum.
Even though the Department of Public Health closed the Fossum case, the Los Angeles County Sheriff's Department has decided to take another look. A detective there says they opened a homicide investigation into her death earlier this year.