Two men imprisoned for killing a California gas station manager are trying to get their cases overturned by arguing that Los Angeles County investigators broke the law when they had Google scour location data for millions of devices in search of potential suspects.

The appeal is part of a growing attempt by defense lawyers and privacy advocates to curtail police use of geofence warrants, an investigative tool powered by the public’s reliance on phones that track their movements.

Driving the resistance is concern that the warrants give police too much discretion in deciding where to search and whose movements seem suspicious. Opponents say the warrants violate the U.S. Constitution’s protections against unreasonable searches by combing through the location data of innocent Google users in search of possible suspects. They also point to cases in which geofence warrants led police to the wrong people: a bicyclist swept into a burglary investigation, a warehouse worker mistakenly charged with murder.