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A San Diego doctor first started asking questions about his mother's care after she died in a California hospital and staff seemed to be hiding a medical mistake.

Diane Stewart had her knee replaced at Stanford Hospital but died shortly after having surgery.

Under oath, Stewart's nurse admitted there was information missing from her medical file.

"This is a tragedy that not only could have been prevented, it should have been prevented," said Chris Dolan, the lawyer for the Stewart family.

Dolan told 10News he was investigating what he thought was a medical malpractice case when he said he found doctored records.

"The reason why these records get destroyed, altered, deleted is because people want to cover their butt," he said.

Stewart's medical records were missing during a critical time in her care, which was just before she died.

The California Department of Public Health investigated and found her records failed to document 'a blood pressure reading' and failed to document 'a care plan to address… (her) abdominal pain.'

The state concluded that some of Stewart's medical records "had been deleted."

Stanford Hospital said the staff involved has been re-trained.

"It can't be mere coincidence," Dolan said. "I've been doing this 17 years and I've come across a dozen cases where the medical records were either altered or deleted."

Dolan led the efforts to make it illegal to change or delete electronic medical records without keeping an original. Though the law is in effect, there are questions about enforcement.

The Department of Public Health only has the power to issue a written warning if they catch someone altering medical records. The department's spokesman told 10News, "Alteration or deletion of medical records in hospitals [is] rare."

Dolan said a slap on the wrist is much cheaper than a lawsuit.

"I heard the words 'I don't know' from some of the most highly educated people more than I've ever heard the words 'I don't know' used in my life before," he said. "The records of what occurred during the span disappeared."