* Cell phones contain a mass of personal data about owner
* U.S. Constitution prohibits unreasonable searches
* Arguments set for April, ruling expected by end of June (Adds context, details about cases, paragraphs 5-21)
WASHINGTON, Jan 17 (Reuters) - The U.S. Supreme Court agreed on Friday to decide whether police can search an arrested criminal suspect's cell phone without a warrant in two cases that showcase how the courts are wrestling to keep up with rapid technological advances.
Taking up cases from California and Massachusetts arising from criminal prosecutions that used evidence obtained without a warrant, the high court will wade into how to apply older court precedent, which allows police to search items carried by a defendant at the time of arrest, to cell phones.
Cell phones have evolved from devices used exclusively to make calls into gadgets that now contain a bounty of personal information about the owner.
The legal question before the justices is whether a search for such information after a defendant is arrested violates the Fourth Amendment of the U.S. Constitution, which bans unreasonable searches. The outcome would determine whether prosecutors in such circumstances could submit evidence gleaned from cell phones in court.
Digital rights activists have sounded the alarm about the amount of personal data the government can now easily access, not just in the criminal context, but also in relation to national security surveillance programs.
President Barack Obama on Friday announced plans to rein in the vast collection of Americans' phone data in a series of limited reforms prompted by disclosures by former National Security Agency contractor Edward Snowden about the sweep of U.S. eavesdropping activities.
Stanford Law School professor Jeffrey Fisher, who represents one of the defendants, said in court papers that it was important for the high court to decide the issue.
"In light of the frequency with which people are arrested with cell phones and the judiciary's confusion over whether the police may search the digital contents of those phones, this court's intervention is critical," Fisher said.
According to a 2013 report by the Pew Research Center, 91 percent of adult Americans have a cell phone, more than a half of which are smartphones that can connect to the Internet and contain personal data from social media websites and other sources.
Under court precedent, police are permitted to search at the time of an arrest without a warrant, primarily to ensure the defendant is not armed and to secure evidence that could otherwise be destroyed. In the past, it has applied to such items as wallets, calendars, address books and diaries.
In the case involving Fisher's client, David Riley was convicted of three charges relating to an August 2009 incident in San Diego in which shots were fired at an occupied vehicle.
Prosecutors linked him to the crime in part due to a photograph on his smartphone that showed him posing in front of a car similar to one seen at the crime scene. The photograph was shown to the jury at trial, as were videos that showed Riley making gang-related comments.
Police searched the phone after pulling over Riley's car for having expired tags 20 days after the shooting. Riley sought the high court's review after his convictions were upheld by a state appeals court in California.
In the other case, the federal government appealed after an appeals court threw out two of three drugs and firearms counts on which Brima Wurie had been convicted by a jury in Massachusetts.
The Boston-based 1st U.S. Circuit Court of Appeals said in a May 2013 ruling that police could not search Wurie's phone without a warrant after the September 2007 arrest for suspected drug dealing.
One major difference between the cases is that Wurie's phone, unlike Riley's, is not a smartphone. Officers used the phone only to search the phone log. They were able to find a phone number that took them to Wurie's house in Boston, where drugs, a gun and cash were found.
The Electronic Frontier Foundation, a digital civil rights group based in San Francisco, urged the court to hear the case in a friend-of-the-court brief.
"I think it's another opportunity for the court to decide how changes in technology implicate the Fourth Amendment," one of the group's lawyers, Hanni Fakhoury, said in an interview.
Fakhoury, a former federal public defender, said that searches of cell phones upon arrest are routine in the vast majority of jurisdictions nationwide.
The U.S. Justice Department defended the practice in its brief urging the court to hear the Wurie case. Government lawyers say searching a cell phone is no different than searching other items commonly found on a person at the time of arrest.
A prohibition against the searches would be particularly troublesome to prosecutors because of "the ubiquity of cell phone use by drug traffickers and other serious offenders," the government lawyers said.
The court will hear oral arguments in April and issue rulings by the end of June. The cases are Riley v. California, 13-132 and U.S. v. Wurie, 13-212. (Reporting by Lawrence Hurley; Editing by Will Dunham and James Dalgleish)