San Jose, California police have charged 90-year-old Anthony (Tony) Vincent Aiello in the gruesome death of his 67-year-old step-daughter last week after obtaining dramatic health data recorded by her Fitbit Alta HR device.

The step-daughter, Karen Navarra, was found dead in her house on Thursday September, 13, from multiple, deep, and instantly incapacitating skull wounds likely inflicted using a small hatchet or axe, according to a police report and autopsy. Her Fitbit data indicated the exact moments those wounds occurred, with her heart rate dramatically spiking then crashing to nothing. The timing of that plummet led to Aiello’s arrest.

A co-worker made the initial, grisly discovery of Navarra’s murder while checking in on her after she didn’t show up for her job as a pharmacy technician at the Regional Medical Center, where she had worked for 45 years. Police arriving on the scene reported that they found Navarra slouched in a chair at her dining room table, covered in blood, and surrounded by blood-splattered surfaces and curtains.