A 90-year-old man in California has been charged in the slaying of his 67-year-old stepdaughter after police alleged his alibi was untrue based on evidence from her fitness tracker.

On Sept. 13, a co-worker of pharmacy technician Karen Navarra went to the woman’s San Jose home to conduct a welfare check when she failed to show up for work. Inside the residence, the co-worker found Navarra’s bloodied body slumped over a chair in the kitchen.

When homicide investigators arrived on the scene, they observed “multiple deep and intrusive wounds” on Navarra’s head and a “gaping laceration” on her neck, according to a report by San Jose police. Navarra’s right hand was clasping a large kitchen knife, which authorities suspect was planted in an attempt to stage a suicide.

A subsequent autopsy determined Navarra’s injuries could not have been self-inflicted and are consistent with injuries caused by a hatchet or ax, police said.

Anthony “Tony” Aiello, who is married to Navarra’s 92-year-old mother, is believed to be the last person to see her alive. Questioned by police, Aiello allegedly said he brought Navarra pizza and biscotti on Sept. 8 and visited with her for about 15 minutes before leaving.

Aiello further claimed he saw Navarra drive by his house later that day, according to court documents. Aiello allegedly said he’d seen someone with her, seated in the passenger side of the vehicle.

San Jose Police Department Authorities have identified Anthony Aiello as a suspect in the slaying of Karen Navarra.

But in piecing together the evidence, police began to suspect Aiello’s version of events could not be true.

Cops say Aiello’s alibi began to fall apart when they compared surveillance video to data collected from Navarra’s Fitbit, a wearable device that measures a person’s fitness metrics and heart rate.

Navarra’s Fitbit, authorities said, recorded a “significant spike” in her heart rate at 3:20 p.m. local time on Sept. 8. The spike was allegedly followed by a rapid slowing of her heart rate, which the device stopped registering at 3:28 p.m., police said.

Surveillance footage recorded outside Navarra’s residence on Sept. 8 allegedly shows Aiello’s car was parked in her driveway from 3:12 p.m. to 3:33 p.m., indicating he was present at the time of her death, police said. The same video footage allegedly shows Navarra did not leave her residence after Aiello’s visit – meaning she could not have driven by his house, police said.

“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,” court documents read. “Aiello stated that could not be true because she had walked him to the door when he left the residence.”

Aiello allegedly told investigators “someone else might have been in the house,” police said.

Rich Fury via Getty Images Fitbits on display in Los Angeles, California, on July 28, 2018.

Police are citing other evidence as well. When authorities conducted a search of Aiello’s residence they allegedly found two shirts with blood spatter. Aiello, according to court documents, told investigators he might have cut his hand while wearing those shirts.

“He was told that the deposits of blood were not localized in one area and were more consistent with splatter,” court documents read. “Aiello indicated that he might have cut his hand and shaken it while he was wearing those shirts.”

Authorities have not commented on a possible motive in the slaying.

Aiello was arrested Sept. 25 and booked on suspicion of murder. He was remanded to the Santa Clara County Jail, where he’s being held without bond, pending a Thursday court appearance.

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