The personal activity-tracking device, Fitbit, has become increasingly popular since it was first released in 2009. It was therefore only a matter of time until one would be worn by victims or suspects of crimes — and potentially be the key to finding out what had actually occurred. As Lauren Smiley explains for Wired, people are inadvertently wearing “a sort of black box for the body that reveals physiological truths that its wearer might prefer to conceal.”

At 90 years old, Tony Aiello was arrested for the murder of his stepdaughter, Karen Navarra. On the day of her death, Tony dropped by her house with a surprise treat of biscotti and pizza. He claimed she was alive when he left. Her Fitbit told a different story.

At a San Jose police station, Tony was hauled into a homicide interrogation room. “What the hell am I doing here?” he asked detectives Brian Meeker and Mike Montonye. Then he waived his right to remain silent and amiably rattled off his life history and answered questions about Karen, until one detective abruptly shifted the subject: Did Tony know what a Fitbit was? He shook his head. They told him that it was a watch with a step counter built into it. “Oh, nice,” Tony marveled, not seeing where the questioning was going. It also has a heart rate monitor, they said. “Oh, that’s better yet.”

The detectives continued: The data shows that Karen’s heart stopped at 3:28 pm, they told Tony. What’s more, they knew Tony was there at the time.

“Oh, no,” Tony said. “She was alive when I left.”

Should we be using Fitbit information in court cases? There are still no set legal standards for how and when this new type of data should be admitted. There are also no guarantees it can be relied on.

Smartwatches decipher heart rate using green LEDs that beam hundreds of times per second into capillaries through the skin. Those capillaries allow in more of the light when full of blood, and less between beats, and the device measures how much light is absorbed. That measurement is then siphoned through a proprietary algorithm to generate a heart rate figure. University of Wisconsin researchers looked at how well wrist-worn fitness trackers measured heart rate, comparing it to an electrocardiograph, the gold standard for heart monitoring. They found that the fitness trackers’ heart rate deviated more from the actual rate when a subject exercised on a treadmill than when at rest. (Fitbit won’t talk specifics about its accuracy, saying in a statement, “We are confident in the performance of all our devices” and that the company continues to test them.)