The last time Anthony Aiello spoke to his stepdaughter, he took homemade pizza and biscotti to her house in San Jose, Calif., for a brief visit. Mr. Aiello, 90, told investigators that she then walked him to the door and handed him two roses in gratitude.
But an unnoticed observer in the house later revealed that their encounter ended in murder, a police report said.
Five days afterward, Mr. Aiello’s stepdaughter, Karen Navarra, 67, was discovered by a co-worker in her house with fatal lacerations on her head and neck. She had been wearing a Fitbit fitness tracker, which investigators said showed that her heart rate had spiked significantly around 3:20 p.m. on Sept. 8, when Mr. Aiello was there.
Then it recorded her heart rate slowing rapidly, and stopping at 3:28 p.m., about five minutes before Mr. Aiello left the house, the report said.
Mr. Aiello was arrested last week on murder charges and booked into the Santa Clara County Jail, the San Jose Police Department said. On Thursday, he will appear in court in the Hall of Justice in San Jose, according to the Santa Clara County district attorney’s office.
While originally intended to motivate people to take control of their fitness and health, fitness devices have found their way into the technology toolbox that law enforcement experts use to solve crimes, alongside videos, GPS devices and cellphones.
Fastened to a person’s body, the devices have a unique front-row seat to their hosts’ lives, inadvertently documenting both mundane and perilous encounters as they record heartbeats, sleeping patterns and physical exertion.
Fitbit location data factored into a sexual assault case in Pennsylvania in 2015 and a personal injury case in Canada in 2014. A Garmin Vivosmart GPS recorded a woman’s struggle with an attacker in Seattle in 2017. The same year, investigators used data from the Fitbit of a Connecticut woman to charge her husband with murder.
This year, investigators in Iowa, with the help of F.B.I. experts, sifted through data from the Fitbit of Mollie Tibbetts, a 20-year-old student who was missing for about a month before her body was discovered in August. Surveillance video led them to a 24-year-old man who was charged with murder.
“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.
When Ms. Navarra’s Fitbit data was compared with video surveillance from her home, the police report said, the police discovered that the car Mr. Aiello had driven was still there when her heart rate stopped being recorded by her Fitbit.
Bloodstained clothes were later found in Mr. Aiello’s home, the document said. He was arrested on Sept. 25.
Mr. Aiello was “confronted” with the Fitbit information during questioning, said Brian Meeker, a San Jose police detective. “After explaining the abilities of the Fitbit to record time, physical movement, and heart rate data, he was informed that the victim was deceased prior to his leaving the house,” Detective Meeker said in the report.
Mr. Aiello said that could not be true, insisting Ms. Navarra had walked him to the door, and he suggested that someone else could have been in the home, the report said.
“I explained that both systems were on internet time, and there was no deviation,” Detective Meeker said.
After they finished their questions, detectives left Mr. Aiello alone in the interview room. He began talking to himself, the report said, saying repeatedly, “I’m done.”