A man has been charged on suspicion of murdering his stepdaughter after her Fitbit showed her heart rate spiking and flatlining while he visited her.
Anthony Aiello, 90, has been accused of murdering Karen Navarra, 67, who was found in her house with fatal cuts to her neck.
Police are examining Miss Navarra's Fitbit Alta HR, a wearable tech wristband which monitors her vital signs and movement, because it appears to disprove Mr Aiello's alibi.
Her heart rate spiked at 3.20pm on September 8 before rapidly slowing and her last heart beat was recorded at 3.28pm that day.
According to surveillance video obtained by police in San Jose, California, Aiello's car was parked outside her house between 3.12pm and 3.33pm that day.
He was arrested over the murder and has appeared in court where he did not enter a plea.
Fitbits have been marketed at joggers but they are increasingly allowing law enforcement an invaluable window into the lives of victims of crime.
Aiello's case has been dubbed the 'Fitbit murder' and the investigation began when a co-worker found Miss Navarra at her home with her throat cut on September 13 because she had not been seen for days.
A knife had been put in her hand to suggest it was suicide.
Aiello told police he visited her home to take her homemade pizza and biscotti but left after 15 minutes.
He claimed she walked him to the door and gave him two roses to thank him.
Detectives were able to piece together a different story using the surveillance footage at her home and the data from her Fitbit, which she wore on her left arm.
The device was connected via Bluetooth to her computer which meant that the data was logged on Fitbit's servers.
Bloodstained clothes were also found in Aiello's home, police said in a report.
In the report Brian Meeker, a San Jose police detective, said that they confronted Aiello with the Fitbit data during questioning but he insisted that he was not the killer
Detectives left Aiello in the room alone when he began talking to himself, saying: "I'm done."
The case comes after police in Iowa used movement data from a Fitbit to solve the killing of student Mollie Tibbetts, who was slain after going for an evening jog.
In 2016 police in Bentonville, Arkansas confiscated an Amazon Echo and issued a warrant to the tech giant for its recordings in case it could help them with a murder inquiry.
The victim also had a smart water meter which indicated 140 gallons were used on the night of the murder in which the killer strangled him in a hot tub.
Jeff Rosen, the district attorney for Santa Clara County, which is prosecuting Aiello, said: "From doorbell security footage to Fitbit, technology engineered to solve some of life's issues are solving serious crimes.
"We are continually inspired by law enforcement investigators who are thinking outside of the box."
Fitbit declined to comment on the case but shared a copy of its privacy policy, which says in part that the company complies with search warrants and court orders.