Technically, it is legal for a law enforcement agency to track a person’s movements via their cell phone. In fact, if there is a court order, they can access messages, conversations, names, and phone numbers.
The police are using what is known as StingRay devices to track cell phones. There seems to be no limit to the use of these devices when it comes to a police investigation. With a StingRay device, which has been purchased by many police stations in the country, a law enforcer can, in fact, alter a device to force it to drop from a 4G or 3G bandwidth to an older 2G band, making it more vulnerable to tracking.
These facts can be food for conspiracy theories especially after it was found that the NSA allegedly had access to phone calls of millions of Verizon customers - a scandal that took place in 2013. Of course, the fact that a police officer can pin your location using your cell phone and even directly alter its band reception is something that can freak out many cell phone users.
However, police enforcers vow that they only utilize these devices in the line of duty. The Oakland Police Department, the San Francisco Police Department, the Sacramento County Sheriff's Department, the San Jose Police Department, and the Fremont Police Department, all Law Enforcement Agencies in Northern California have purchased StingRay devices. These devices not only help with geolocation but also allow full access to a suspect´s phone.
In most cases, police officers do not need a warrant to locate a cellphone. They do need a court order to access the information on the cell phone, but it has to be clear and proven that this could bring the investigation to a possible solution.