Following on from my post earlier today about law enforcement agencies wanting to break communications encryption, here is another article from The Verge about police in Baltimore just breaking the mobile phone system in trying to investigate cases.
It seems that Baltimore police have used the StingRay phone tracker at least 25,000 times over the past eight years. This is a device (based on my quick read of the Wikipedia page) that is designed to download information from all mobile phone in an area and, as a side effect, disrupts service to the cellular network.
As far as I can tell, this sort of device is common throughout the United States and probably in many other countries. I believe that the use of this sort of device is an unacceptably broad search of the population.
Just merely allowing police departments access to these devices means that they will be used and likely used excessively. When the only tool you've got is a hammer, everything looks like a nail.