“From doorbell security footage to Fitbit, technology engineered to solve some of life’s issues are solving serious crimes,” said Jeff Rosen, the district attorney for Santa Clara County. “We are continually inspired by law enforcement investigators who are thinking outside of the box.”

In the San Jose case, the police said their investigation used a combination of video surveillance and data from Ms. Navarra’s Fitbit, an Alta HR device, which she wore on her left wrist and synchronized with a computer in her home, where she lived alone.

On Sept. 13, a co-worker of Ms. Navarra’s went to the house to check on her because she had not showed up for her job at a pharmacy, the report said. The front door was unlocked, and she discovered Ms. Navarra dead, slouched in a chair at her dining room table.

She had lacerations on her head and neck, and a large kitchen knife was in her right hand, the report said. Blood was spattered and uneaten pizza was strewn in the kitchen. The coroner ruled the death a homicide.

Detectives then questioned Ms. Navarra’s only known next-of-kin, her 92-year-old mother, Adele Aiello, and Mr. Aiello. Mr. Aiello told the authorities he had dropped off the food for his stepdaughter and left her house within 15 minutes, but he said he saw Ms. Navarra drive by his home with a passenger in the car later that afternoon.

Investigators obtained a search warrant and retrieved the Fitbit data with the help of the company’s director of brand protection, Jeff Bonham, the police report said.

On Wednesday, Fitbit declined to comment on the case but shared a copy of its privacy policy, which says in part that the company complies with legal processes, including search warrants and court orders, when it shares data.