One of the most important witnesses to the rape and homicide of a 19-year-old-woman in Germany might be a stock app on the iPhone of her alleged murderer.

Hussein K., an Afghan refugee in Freiburg, has been on trial since September for allegedly raping and murdering a student in Freiburg, and disposing of her body in a river. But many of the details of the trial have been hazy—no one can agree on his real age, and most notably, there’s a mysterious chunk of time missing from the geodata and surveillance video analysis of his whereabouts at the time of the crime.

He refused to give authorities the passcode to his iPhone, but investigators hired a Munich company (which one is not publicly known) to gain access his device, according to German news outlet Welt. They searched through Apple’s Health app, which was added to all iPhones with the release of iOS 8 in 2014, and were able to gain more data about what he was doing that day. The app records how many steps he took and what kind of activity he was doing throughout that day.

The app recorded a portion of his activity as “climbing stairs,” which authorities were able to correlate with the time he would have dragged his victim down the river embankment, and then climbed back up. Freiburg police sent an investigator to the scene to replicate his movements, and sure enough, his Health app activity correlated with what was recorded on the defendant’s phone.