In Pennsylvania in 2015, the Fitbit data of a woman who said she was sexually assaulted by an unknown man in the home she was visiting contradicted her statement. While she told police she was sleeping, the device recorded her walking around all night.

In 2015, a Connecticut murder victim’s Fitbit showed that she was walking around her house at the same time her husband would later claim she was being attacked by an outside intruder. The number of steps the Fitbit recorded contradicted the husband’s version of the story. The other evidence in the case included Facebook messages and alarm system records that showed movement in the house that also went against his account. He remains in jail awaiting trial.

Accounts of this last case show how much police could piece together of the hour during which the murder took place, just from electronic evidence, and the couple’s online life: the Fitbit shows when the wife left the gym and arrived at home, that she posted to Facebook from an IP address associated with the house, that the husband checked the gym’s schedule online, emailed his boss that he’d be late to work, and when exactly the house alarm was triggered.

For law enforcement, Fitbit data is like an “electronic footprint,” Craig Stedman, the district attorney in the Pennsylvania case, told the Hartford Courant. “We can also get the information much faster than some other types of evidence such as DNA tests.”

In the San Jose case, detectives obtained a warrant and had a Fitbit executive come down to retrieve the device himself. They were able to get the information the next day.

Law enforcement will only have more and more health data to access. Smartwatches are becoming more popular and more sophisticated in the data they collect. In Australia, Apple Watch data was used earlier this year to arrest a murder suspect, another case of a family member allegedly staging an attack only to have their story contradicted by the victim’s health-tracking device.

iPhones have a built in health app, which, once activated, calculates steps and measures your heart rate. This data was used in at least one criminal case, in Germany. The suspect’s phone data showed “strenuous” activity that investigators linked to dragging a body and climbing up stairs.

Just as these tools are powerful, their use by law enforcement can be concerning. For one, data from fitness trackers is often inaccurate, both in terms of location data and heart rate measurements.

Judges, prosecutors, or defense attorneys may not be fluent in how the technology works, which makes it harder for them to thoroughly evaluate evidence, notes the Rand Corporation in a 2017 report. Their understanding of the technology affects, in turn, how well they can instruct the jury.

And as the technology develops, other problems might arise, the report points out, along with questions about what the law does protect: How do you treat a device planted under the skin, for example? Does it become a part of a person, or is it a device subject to a search warrant?

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