That's a question the Supreme Court will answer as it begins hearing opening arguments Wednesday in what is being called a "major test of privacy rights."
Cellphone data is useful for criminal investigators because it can track the location and movement of cellphone users over extended periods.
But the outcome of the Supreme Court case could have other, far-reaching implications. For example, it could impact how the government exercises surveillance power over the public. In this age of ever-increasing digital technology, questions arise about whether online medical records, Google searches and even smartwatch data might become fair game for investigators.
What prompted this case?
Nine armed robberies at T-Mobile and Radio Shack stores in Ohio and Michigan in 2011 eventually led to the Supreme Court case. One of the robbers confessed to the crimes and gave the FBI his cellphone number, along with the numbers of others allegedly involved in the crime spree.
The FBI obtained cell tower data for Timothy Carpenter, placing him in the area of the robberies. In part, the cellphone data led to his conviction on aiding and abetting the crimes.
Defense lawyers for Carpenter argued that the government needed but failed to obtain a "probable cause" search warrant to get the data.
A federal appeals court ruled in favor of the government, saying no court has yet decided if the Fourth Amendment protections apply to "internet analogue to envelope markings, namely the metadata used to route internet communications, like sender and recipient addresses on email, or IP addresses."
Why are privacy advocates worried?
The ACLU stepped in, appealing the case to the Supreme Court.
Using cellphone data to trace users over time is a practice that concerns privacy advocates and staunch supporters of the Fourth Amendment.
The Electronic Frontier Foundation (EFF), a nonprofit privacy advocacy group, wants the practice to be found unconstitutional. The group says it violates privacy protections provided under the Fourth Amendment that protect citizens against unreasonable searches and seizures by the government. The EFF has said the cellphone location data can provide an "incredibly detailed picture of people's private lives."
"Cell phone users don't voluntarily provide location data to their providers, it happens automatically without their control and is generated whether or not the phone is being used," EFF Senior Staff Attorney Jennifer Lynch stated on the group's website. "Other federal courts and judges in several states have recognized that the so-called third party doctrine doesn't apply to CSLI (cell site location information). It's time for the Supreme Court to consider whether a decision it made before the existence of commercial cell phones, which are now ubiquitous and reveal our every move, can still be used to override Fourth Amendment protections."