A woman’s Fitbit clocked a rapid rise in her heart rate before a sudden drop-off to nothing, helping San Jose police piece together clues and charge her 90-year-old stepfather with her murder, according to court records.

Tony Aiello was arrested Wednesday on suspicion of murdering his 67-year-old stepdaughter, Karen Navarra, on Sept. 8. Aiello is married to Navarra’s 92-year-old mother.

Navarra, a pharmacy technician described by family as a “recluse,” was discovered deceased by a co-worker Sept. 13 after she failed to show up to her job.

Police said Navarra was found slumped over in a chair, clutching a large kitchen knife with a “gaping” slit to her neck. To investigators, the scene appeared to be staged to look like a suicide.

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But a later autopsy revealed “multiple deep and intrusive wounds” to her head and facial area, most likely inflicted by a small hatchet or ax. The doctor told investigators Navarra couldn’t have inflicted the wounds herself and classified the case as a homicide.

At his initial interview, Aiello told San Jose police that he dropped by Navarra’s home on Saturday, Sept. 8, to bring her pizza, according to court records. He also said he was outside his own home a few hours later, and witnessed his step-daughter drive by and honk with someone else in the passenger seat.

But police say Aiello’s account didn’t match up with what they saw on surveillance footage.

Nearby cameras captured his car at Navarra’s home for at least 21 minutes that day, between 3:12 p.m. and 3:33 p.m, but they didn’t show her car ever leaving in the direction Aiello described, police said.

Investigators noted Navarra was wearing a Fitbit, a wristband device that can collect data like number of steps walked and heart rate, when she was found, and they later consulted with the company to see if the device could offer any clues. A company employee told police Navarra’s heart rate spiked at 3:20 p.m. Sept. 8 and then rapidly slowed. The device stopped registering a heartbeat at 3:28 p.m.

Aiello repeated his initial story after his arrest and denied that he killed his stepdaughter, according to court records. When confronted with the Fitbit evidence Aiello suggested someone else could have been in the home. He told police she couldn’t have died when he was there, because she walked him to the door.

Police also found clothes in his hamper covered in blood splatter. Aiello told police he cuts himself frequently, according to court records.

Homicide investigators across the country have turned to fitness trackers to help piece together timelines, movements and other key information for their cases.

A Connecticut man was arrested after his wife’s Fitbit data showed her last movements were more than an hour after her husband said she had been killed by intruders.

More online: How Vallejo police botched a kidnap-for-ransom case before falsely declaring it a hoax. http://bit.ly/VallejoKidnap

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In Wisconsin, another man’s Fitbit acted as an alibi. The device corroborated his story during a crucial window of time when police said his live-in girlfriend’s body was being dumped in a field.

Aiello is being held without bail and is due in court Oct. 4.

Megan Cassidy is a San Francisco Chronicle staff writer. Email: megan.cassidy@sfchronicle.com Twitter: @meganrcassidy