Last week, California's Supreme Court reached a controversial 5-2 decision in People v. Diaz (PDF), holding that police officers may lawfully search mobile phones found on arrested individuals' persons without first obtaining a search warrant. The court reasoned that mobile phones, like cigarette packs and wallets, fall under the search incident to arrest exception to the Fourth Amendment to the Constitution.

California's opinion in Diaz is the latest of several recent court rulings upholding warrantless searches of mobile phones incident to arrest. While this precedent is troubling for civil liberties, it's not a death knell for mobile phone privacy. If you follow a few basic guidelines, you can protect your mobile device from unreasonable search and seizure, even in the event of arrest. In this article, we will discuss the rationale for allowing police to conduct warrantless searches of arrestees, your right to remain silent during police interrogation, and the state of mobile phone security.

The Fourth Amendment's search incident to arrest exception

It has long been established under common law that law enforcement officers may conduct warrantless searches of criminal suspects upon arresting them. Courts have identified two exigencies that justify warrantless searches of suspects incident to arrest.

First, the government has a compelling interest in ensuring that detained suspects are not in possession of weapons or other dangerous items. Requiring that police obtain a warrant before determining whether an arrested individual is armed would subject officers to potentially life-threatening risks.

Second, the government has a compelling interest in preventing arrestees from destroying or tampering with evidence of criminal activity in their immediate possession at the time of arrest. Imposing a warrant requirement on police searches of arrestees would afford suspects an opportunity to destroy any incriminating evidence on their persons.

Unfortunately, courts have expanded the scope of this once-narrow exception to create a gaping hole in the Fourth Amendment. In 1973, the United States Supreme Court held in US v. Robinson that warrantless searches of arrestees’ persons are presumptively reasonable and require "no additional justification" to be lawful. In 1974, the Court further held in US v. Edwards that objects found in an arrestee's "immediate possession" may be subject to delayed warrantless search at any time proximate to the arrest—even absent exigent circumstances.

In 1977, the Supreme Court clarified the search incident to arrest exception in US v. Chadwick, holding that the warrantless search of a footlocker found in the possession of criminal suspects violated the Fourth Amendment because the search took place after the suspects had been put into custody and the footlocker had been secured by police. In Chadwick, the Court held that while warrantless searches of objects found on arrestees' persons are presumptively lawful due to the "reduced expectations of privacy caused by the arrest," closed containers that are not "immediately associated with" arrestees' persons are not subject to a delayed warrantless search, barring exigent circumstances.

Based on these precedents, California's Supreme Court held in Diaz that mobile phones found on arrestees' persons may be searched without a warrant, even where there is no risk of the suspect destroying evidence. Therefore, under Diaz, if you're arrested while carrying a mobile phone on your person, police are free to rifle through your text messages, images, and any other files stored locally on your phone. Any incriminating evidence found on your phone can be used against you in court.

On the other hand, if you are arrested with a mobile phone in your possession but not immediately associated with your person, police may not search your phone without a warrant once you’ve been taken into custody and your phone is under police control.

The takeaway from Diaz, therefore, is that you should store your mobile phone in your luggage, footlocker, or in some other closed container that's not on your person, particularly when driving an automobile. (For more on this subject, see our 2008 article summarizing the search incident to arrest exception in the context of mobile phones. Also see The iPhone Meets the Fourth Amendment, a 2008 UCLA Law Review article by law professor Adam Gershowitz.)

What about password-protected mobile phones?

While the search incident to arrest exception gives police free rein to search and seize mobile phones found on arrestees’ persons, police generally cannot lawfully compel suspects to disclose or enter their mobile phone passwords. That's because the Fifth Amendment's protection against self-incrimination bars the government from compelling an individual to divulge any information or engage in any action considered to be "testimonial"—that is, predicated on potentially incriminating knowledge contained solely within the suspect's mind.

Individuals can be forced to make an incriminating testimonial communication only when there is no possibility that it will be used against them (such as when prosecutors have granted them immunity) or when the incriminating nature of the information sought is a foregone conclusion. (For more on this subject, see this informative article forthcoming in the Iowa Law Review, also by Professor Gershowitz, which explores in great depth the uncharted legal territory surrounding password-protected mobile phones seized incident to arrest.)

As such, if you are arrested or detained by a law enforcement officer, you cannot lawfully be compelled to tell the officer anything other than your basic identifying information—even if the officer has not read you the Miranda warning. Exercising your right to remain silent cannot be held against you in a court of law, nor can it be used to establish probable cause for a search warrant.

However, if you voluntarily disclose or enter your mobile phone password in response to police interrogation, any evidence of illegal activity found on (or by way of) your phone is admissible in court, regardless of whether or not you've been Mirandized.

What if you're not a criminal and think you have nothing to hide? Why not simply cooperate with the police and hand over your password so that you can get on with your life?

For one thing, many Americans are criminals and they don't even know it. Due to the disturbing phenomenon known as "overcriminalization," it's very easy to break the law nowadays without realizing it. A May 2010 study from the conservative Heritage Foundation and the National Association of Criminal Defense Lawyers found that three out of every five new nonviolent criminal offenses don't require criminal intent. The Congressional Research Service can't even count the number of criminal offenses currently on the books in the United States, estimating the number to be in the "tens of thousands."

What's more, the US Supreme Court has held that police may arrest you for simple misdemeanors, such as driving without a seatbelt or having unpaid parking tickets. While police don't typically arrest individuals for such trivial infractions, all it takes is one unlucky police encounter and you could end up behind bars. If that happens, and your mobile phone is on your person, it may be subject to a warrantless search. If police dig up an incriminating text message, e-mail, or errant image file on your mobile device, it might be enough to convince a judge to issue a search warrant of your property—or, worse, lead to criminal charges being filed against you.