Next week, the U.S. Supreme Court is expected to hear arguments over whether police can search a person's cellphone without a warrant upon arrest. That will give the justices a rare opportunity to draw a bright line about what police can do in the digital age.

The court should conclude that searching smartphones should require a warrant. That is what law enforcement needs in most cases to search a home.

More than 50 percent of Americans now carry smartphones, and those phones' search histories, photos, emails, chats and contacts offer not only a window on the owner's mind, but also can document their every step and communication.

Courts have given mixed rulings how the Fourth Amendment, which protects people from unreasonable search and seizure, applies to cellphones. The right to privacy in these cases conflicts with the important public interest in police solving and preventing crimes.

The high court will have to balance these two interests in a decision that makes sense not just for smartphones, but also for tablets, laptops and the new gadgets down the road.

"Some members of the court will certainly try to consider the place mobile devices play in modern life," said Jennifer Granick, director of civil liberties at the Stanford Center for Internet and Society.

The justices' ruling will involve two cases. In 2007, Massachusetts police searching a man's rudimentary flip phone noticed a phone number that led to the suspect's home where they found drugs, cash and guns. The 1st Circuit Court of Appeals agreed that the search violated his Fourth Amendment rights. The government is appealing the decision.

In 2009, San Diego police stopped David Riley for expired registration tags. A search of his smartphone revealed images tying him to a gang shooting and other evidence. Convicted of attempted murder and serving a 15-year sentence, Riley has challenged the evidence police found on his cellphone. In a 5-2 decision, the California Supreme Court upheld the cellphone search in the case.

In taking on the Riley case, the high court said it would decide the narrow question of whether evidence admitted at Riley's trial as part of the smartphone search violated the Fourth Amendment.

Legislative attempts to clarify the rules in California have not succeeded. In 2011, state Sen. Mark Leno, D-San Francisco, proposed a bill that would have required a warrant to search cellphones. Gov. Jerry Brown vetoed it.

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Quinn: Supreme Court should clarify Fourth Amendment rights in the digital age