The U.S. Supreme Court may answer part of that question after hearing argument in a California case this spring, though the justices severely narrowed the scope of the dispute. A separate case involving the federal government and a search of an old technology cellphone in Massachusetts also was accepted for Supreme Court argument and review.
The Fourth Amendment's ban on "unreasonable" searches protects the innocent and the guilty, anyone under suspicion. Otherwise, the guarantee would be meaningless.
Police can obtain a search warrant when they have "probable cause" to believe a crime has been committed. But there are emergency circumstances under which officials can conduct a search without a warrant -- if someone's life seems to be in danger or if important evidence is liable to be destroyed.
If one is arrested for any reason, and a cellphone is on the person or on the seat in a vehicle, do police have to show probable cause and get a search warrant before looking at its contents?
A May article in Ad Age said smartphone users are now the majority among people with mobile phones -- more than 50 percent of U.S. mobile phone users have smartphones, according to a report by research firm eMarketer.
The outcome in the high court cases could affect more than cellphones -- extending to other wireless devices such as tablets and laptops. The Washington Post reported in October 2011 there were more wireless devices -- 327.6 million -- that year in the United States than there were people.
In the case out of California accepted by the justices, a defendant wanted the justices to decide the broad question of "under what circumstances the Fourth Amendment permits police officers to conduct a warrantless search of the digital contents of an individual's cellphone seized from the person at the time of arrest."
That issue proved too broad for the justices, who accepted the case for review but restricted the question to whether "evidence admitted at [the defendant's] trial was obtained in a search of [his] cellphone that violated [his] Fourth Amendment rights."
Early on Aug. 22, 2009, police pulled over a Lexus driven by David Riley -- described in his own petition as a "local college student" -- near his home in San Diego.
An officer pulled the car over because of expired tags, but after finding out that Riley was driving with a suspended license, impounded the car. Under San Diego police policy, an officer is required to do an inventory search of an impounded vehicle.
Police discovered two firearms under the car's hood, and arrested Riley for carrying concealed and loaded weapons.
An officer "seized" Riley's "cellphone from his person" -- a Samsung Instinct M800 smartphone, "a touch-screen device designed to compete with Apple's iPhone, capable of accessing the Internet, capturing photos and videos, and storing both voice and text messages, among other functions."
A police officer, without a warrant, "scrolled through the phone's contents at the scene. He noticed that some words (apparently in text messages and the phone's contacts list) normally beginning with the letter 'K' were preceded by the letter 'C,'" Riley's petition to the high court said. The officer "believed that the 'CK' prefix referred to 'Crip Killers,' a slang term for members of a criminal gang known as the 'Bloods.'"
Police performed a second warrantless search of Riley's smartphone at the police station. Riley wasn't talking, but a detective specializing in gang investigations went through the cellphone and found several photos and videos suggesting Riley was a member of a gang.
He also found a photo of Riley posing with another person in front of a red Oldsmobile owned by Riley that police believed had been used in an earlier gang-related shooting in which no one was hit. Ballistics tests on the recovered firearms linked them to the shooting.
Riley was charged along with two others with shooting at an occupied vehicle, attempted murder and assault with a semiautomatic, all for the benefit of a street gang. The gang charge exposed him to enhanced sentences.
Though his co-defendants pleaded guilty, Riley went to trial, and his lawyers asked a judge to suppress all the evidence obtained from the searches of his cellphone, arguing it had been searched without a warrant and "without any exigency [emergency] otherwise justifying the search."
A state judge rejected the request, but Riley's first trial ended with a hung jury. In a second trial the cellphone evidence was again used, and a jury convicted him of all charges. But under state law, the judge restricted the conviction to the most serious charge, shooting at an occupied vehicle, which normally would draw a sentence of seven years. With the gang enhancement, the sentence was 15 years to life.
A California appeals court affirmed Riley's convictions, and the California Supreme Court rejected review.
In successfully asking the U.S. Supreme Court for review at that level, Riley's petition said, "Courts across the country" are struggling to apply the U.S. Supreme Court's "'search-incident-to-arrest jurisprudence' -- a set of legal rules largely developed decades ago, before the digital era" to the question of whether the Fourth Amendment permits warrantless searches "of data on a cellphone seized from the person."
A joint friend-of-the-court brief from the Center for Democracy and Technology -- a Washington non-profit -- and the Electronic Frontier Foundation -- a civil liberties non-profit in San Francisco -- urged the justices to take the case.
The U.S. Supreme Court "repeatedly has recognized the importance of carefully assessing the nature and effects of new technologies in determining whether a government search is 'unreasonable' within the meaning of the Fourth Amendment," the joint brief said.
"This case presents a question regarding the impact of new technology in a context familiar to most Americans: today's mobile telephones -- known as 'smartphones' -- which can, and increasingly do, contain the equivalent of millions of physical pages of information, much of it highly personal.
"The particular issue is whether the search-incident-to-arrest exception to the requirement of a warrant and probable cause permits the government to review all of the information stored in a smartphone simply because an individual is carrying the phone when he or she is arrested."
The lower courts have reached conflicting conclusions, the joint brief said.
In opposing review, California officials told the high court in their own brief, which called Riley "a member of a Blood gang called 'Lincoln Park," not all searches of personal possessions required warrants.
"Together, this [U.S. Supreme] Court's precedent stands for the principle that, while the Fourth Amendment prohibits the warrantless delayed search of items that merely had been 'within the arrestee's immediate control,' the search-incident-to-arrest exception to the search warrant requirement permits a delayed warrantless search of the person of the arrestee and of the personal property 'immediately associated with the person of the arrestee,'" the state brief said.
In the second case accepted by the U.S. Supreme Court, the Obama administration asked the justices whether "the Fourth Amendment permits police, without obtaining a warrant, to review the call log of a cellphone found on a person who has been lawfully arrested."
In that case, Brima Wurie was arrested in South Boston in September 2007, then convicted in federal court of being a felon in possession of a firearm and ammunition, and crack cocaine charges. He was sentenced to nearly 22 years in prison.
But a federal appeals court panel majority threw out two of the convictions, saying evidence admitted in his trial had been unconstitutionally obtained from his "flip" cellphone. When the full 1st U.S. Circuit Court of Appeals in Boston refused to rehear the case, the Justice Department asked the U.S. Supreme Court for review.
"As the court of appeals [panel] acknowledged, its decision also creates a square conflict with the Fourth, Fifth and Seventh Circuits and state supreme courts, including the Supreme Judicial Court of Massachusetts, leaving law enforcement officers in that state the task of making sense of conflicting legal rules," the government's brief said. "Particularly given the ubiquity of cellphone use by drug traffickers and other serious offenders, and the important law-enforcement consequences of unsettling search-incident-to-arrest doctrine, the question presented now 'requires an authoritative answer from the [U.S.] Supreme Court.'"
In opposing review, Wurie's brief also cited conflicts in the the lower courts, but reached the opposite conclusion. The brief said U.S. Supreme Court review of the issue should await further developments in the lower courts.
"The Fourth, Fifth and Seventh Circuits and the highest courts of the states of California, Georgia and Massachusetts have upheld warrantless searches of cellphones incident to arrest with varying rationales," the brief said. "The First Circuit [in Boston], the highest courts of the states of Florida and Ohio and federal district courts in California and Florida have rejected warrantless cellphone searches incident to arrest with varying rationales.
"The varied rationales underlying these opinions, the fact that eight United States Courts of Appeals (and most state high courts) have not addressed the issue in precedential opinions, and the rapid rate at which the technology involved is evolving, all [point to] waiting for the lower federal and state courts to further evaluate the array of legal issues and the impact of technological advances involved in this area before this [U.S. Supreme] Court resolves what differences, if any, will remain after further development."