PEORIA, Ill. (WMBD) — A Peoria County jury on Wednesday got a primer on ways police officers can use their cell phones to track their location as well as their movements.

It’s called “geofencing” and it was a key tool in helping solve the Nov. 19, 2018 fatal shooting of Daryl Keller in South Peoria. The tool allowed a Peoria police detective to figure out who was connected to a car which was reported stolen here in Peoria but found in Chicago.

Seth Landwehr of the Peoria Police Department spent more than a hour on the stand during the trial of the alleged shooter, Lionell Harris, 31, explaining how the concept works.

Harris, who is acting as his own attorney, is facing one count of first-degree murder in connection with Keller’s death. If convicted, he faces at least 20 years and possibly decades more behind bars.

He and a fellow detective were in Chicago trying to find a missing car that reportedly was taken from Peoria. That mattered because a similar car was seen near and almost immediately after the shooting.

Armed with only the location of a drug store, the two “stumbled upon” upon the missing Mitsubishi sedan parked on the street.

It had several inches of snow on it, the same type of damage that was seen on the similar car that drove in the 200 block of Sand Street and past cameras near the entrance to the Landmark Apartments.

Landwehr was able to set up a virtual “fence” around the car through Google and then was able to see tens of thousands of devices come through the area. The search engine giant set back a spreadsheet full of data points involving latitude and longitude points, a unique ID number for a person’s mobile device and then time people spent within the designated area.

No personal data, the detective told jurors, was delivered to him. Instead, he was looking at numbers only.

After weeks of looking at each entry, he was able to narrow it down to 11 and from there, one. It was only then was he able to get personal data regarding that device’s Google account, phone number and more.

Using that, he was able to get more information which eventually led to Harris’ arrest.

The story will continue Thursday in the courtroom of Circuit Judge John Vespa.