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Historically, as new technology emerges the law changes and adapts. A recent example is the cell phone, which has become ubiquitous in American society. The applicable law continues to evolve, especially in the area of search and seizure in criminal cases.

Privacy of Cell Phone Records

Most viewers of television police dramas and “true crime” stories know that cell phone records have become important police investigative tools. Both landline and cell phone records have been used for years to show that a particular call was made to or received at a given number, but the location of a traditional phone (or, more significantly, the caller) was of course not typically in doubt.

Now, however, the antenna or “tower” with which a particular cell phone was connected at any time can be determined from carrier records. These “locator” records are routinely offered to prove that a suspect ‒ or at least his or her cell phone ‒ was in a particular area at a given time. In many instances, carriers release these records without the subscriber’s consent or a valid search warrant.

cell phone records and activities

The Fourth Amendment to the United States Constitution guarantees that citizens will be protected against unreasonable searches of their “persons, houses, papers, and effects”. Because the guarantee applies only when the person has a reasonable expectation of privacy, however, the police can observe and even follow a defendant on a street or other public area without first obtaining a search warrant. Evidence recovered from a suspect’s outdoor trash can is another example.

Courts have been obliged to rule on the question of whether a cell phone subscriber has a reasonable expectation that “locator” information is private. If so, a search warrant would normally be required to obtain the carrier’s records.


U.S. Constitutional Case

In the latest of several United States Supreme Court Fourth Amendment cell phone cases, in 2017 the  Court agreed to consider the locator information question in the case of Carpenter v. United States. Carpenter was convicted of participating in several burglaries in Michigan and Ohio. While there was both co-defendant and independent eyewitness testimony placing Carpenter near the scenes of several of the crimes, the prosecution also introduced locator information obtained without a warrant from Carpenter’s cell phone carrier.

The Court heard oral argument in September, and a decision is expected by early Summer 2018. A ruling in Carpenter’s favor would require significant changes in police investigative procedures nationwide. In that event, it is hoped that the Court will also indicate whether the decision must be applied retroactively.


State Law

A number of states have enacted laws that afford some privacy protection to cell phone data. The 2015 California Electronic Communications Privacy Act (also known as CalECPA) requires that a search warrant is required to obtain either cell phone messages or locator information. The legislation was widely supported by both privacy and civil liberties organizations and a number of large tech companies, including Google, Twitter, and Facebook.

According to the American Civil Liberties Union, the passage of CalECPA made California the largest state by far to require a search warrant for all or virtually all cell phone data.


There are a couple of important points to remember about CalCEPA:

  • First, it applies only to California state and local law enforcement agencies. At least until the Carpenter decision is issued, federal investigators need not comply with CalCEPA’s warrant requirement
  • In addition, the Act has exceptions for records disclosed voluntarily by the subscriber and for certain emergency situations


Author Photo

Kerry L. Armstrong


Attorney Kerry Armstrong opened up his law firm in June 2007. Mr. Armstrong attended Thomas Jefferson School of Law, San Diego, California, and received his B.S. from Middle Tennessee State University. Kerry L. Armstrong became certified by the State Bar of California’s Board of Legal Specialization for criminal law in August 2020, making him one of the few criminal defense attorneys with a criminal law legal specialization certificate in San Diego County.  Between 2014 – 2019, Mr. Armstrong was selected for inclusion in the California Super Lawyers list, an honor only awarded to 5% of the nation’s attorneys.

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