Carrying a cellphone creates a "historical log" of movements, privacy expert says

WASHINGTON -- For law enforcement, cellphone data is a powerful new tool. It sent Timothy Carpenter to prison after Michigan police linked Carpenter to a string of armed robberies, retracing his steps as his cellphone connected to different cell sites.

But in a major statement for privacy in a technological age, the Supreme Court on Friday threw out Carpenter's conviction, finding the government violated his rights when it monitored his movements without first getting a warrant. The Supreme Court ruled that police -- in most cases -- need a warrant to track a person's movements by cellphone location.

"When the government tracks the location of a cellphone it achieves near perfect surveillance," wrote Chief Justice John Roberts. "As if it had attached an ankle monitor to the phone's user."

"We all carry our cellphones with us everywhere now and so what that creates is a historical log of your locations and movements overtime every day," said Alan Butler, a privacy expert.

Butler said the government can track you everywhere you go. He said the court finally recognized the intrusiveness of today's technology.

"There has been a seismic shift in technology and also in the implications for privacy since the 1970s when most of these cases were decided," Butler said. "Digital data is just different fundamentally than analog, old-school phone records or bank records."

The chief justice parted ways with the court's four other conservatives, who were all in dissent. Justice Samuel Alito said the ruling will threaten "many legitimate and valuable investigative practices" law enforcement has come to rely on.

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    Jan Crawford is CBS News Chief Political and Legal Correspondent. She is from "Crossroads," Alabama.