Nataline Sarkisyan died Thursday night at about 6 p.m. at University of California, Los Angeles Medical Center. She had been in a vegetative state for weeks, said her mother, Hilda.
"She passed away, and the insurance (company) is responsible for this," she said.
The family is planning to sue the insurance company, their attorney said Friday.
Attorney Mark Geragos said he also plans to ask the district attorney to press murder or manslaughter charges against CIGNA HealthCare for the death of Nataline Sarkisyan.
The insurer "maliciously killed her" because it did not want to bear the expense of her transplant and aftercare, Geragos said at a news conference outside his downtown Los Angeles office. He did not say when or in what court he would file the suit.
Nataline had been battling leukemia and received a bone marrow transplant from her brother. She developed a complication, however, that caused her liver to fail.
Doctors at UCLA determined she needed a transplant and sent a letter to CIGNA Healthcare on Dec. 11. The Philadelphia-based health insurance company denied payment for the transplant.
CIGNA stated her plan did not cover "experimental, investigational and unproven services," her doctors said, reports CBS News station KCBS-TV.
On Thursday, about 150 teenagers and nurses protested outside CIGNA's office in Glendale. As the protesters rallied, the company reversed its decision and said it would approve the transplant.
"This is an incredible turnaround generated by a massive outpouring around the country that proves that an engaged public can make a difference and achieve results," said Rose Ann DeMoro, executive director of the California Nurses Association and National Nurses Organizing Committee, one of the rally's organizers, reports KCBS.
Despite the reversal, CIGNA said in an e-mail statement before she died that there was a lack of medical evidence showing the procedure would work in Nataline's case.
In their letter, the UCLA doctors said patients in situations similar to Nataline's who undergo transplants have a six-month survival rate of about 65 percent.
Officials with CIGNA could not immediately be reached for comment Thursday night.