Like lineups, fingerprints and DNA, cellphones have become routine tools in the fight against crime.

Hundreds of times a day, police or other law enforcement agencies ask America’s telecommunication companies for records showing when someone’s mobile phone pinged cell towers — information they can use to track where the person traveled over days, weeks and months.

Many of those people are suspects in crimes ranging from murder to corruption to stalking. Some are innocent. Others are witnesses or victims.

And in most of those cases investigators didn’t ask a judge for a warrant — a process that would have required them to explain in detail how that information would help them solve the crime.

But they’ll have to do it now — in most cases.

The U.S. Supreme Court ruled Friday that obtaining “cell site location information” without a warrant was an invasion of privacy because that data can be used to monitor someone’s movements 24 hours a day, providing “near perfect surveillance” of a person’s travels — far beyond the narrow purpose of a criminal investigation.

The case concerned a Michigan man named Timothy Carpenter who was convicted in a string of robberies after FBI agents used location data from his cellphone to argue that he was near the crime scenes at the time the holdups happened. Its impact now extends to just about anyone who has a cellphone — more than 95 percent of American adults.

“Everyone should care about this,” said Andrew Crocker, a staff attorney for the Electronic Frontier Foundation, a nonprofit that advocates for digital privacy rights. “If the government doesn’t need warrants, then there’s nothing limiting them to collecting information from just the bad guys.”

Redefining privacy

The ruling provides Fourth Amendment protection against “unreasonable searches and seizures” from one of the most common and invasive government surveillance techniques, lawyers and privacy advocates say. And it could pave the way for restrictions on how law enforcement can seek other kinds of digital data, from internet browsing to use of phone applications to walking under the gaze of surveillance cameras.

The ruling could also set standards for privacy in other aspects of American life where people’s digital activity is mined for information.

“This takes us a lot closer to the European system, which makes personal information sacrosanct,” said Nick Akerman, a former federal prosecutor and cybercrime expert. “It’s really defining what we view as private.”

An avalanche of requests

Seeking cellphone records has become standard in criminal cases in which authorities want to look at someone’s movements — not just to show whether a suspect was near the scene of a crime but also to evaluate leads, verify tips from informants, corroborate witnesses’ stories and even determine the whereabouts of victims, including those who are reluctant to talk or have been killed.

The Electronic Frontier Foundation examined telecommunication companies’ public filings and found they received tens of thousands of requests from law enforcement annually. In 2016 alone, AT&T received more than 70,000 requests, and Verizon handled 53,000, the EFF said in a brief submitted in the Carpenter case.

“It’s incredibly common,” said Michael Price, senior litigation counsel with the National Association of Criminal Defense Lawyers, who also submitted a brief in support of Carpenter’s claim.“It’s something we’ve seen in recent years as one of the go-to investigatory tools that police use before they have enough evidence to arrest somebody or indict somebody.”

Cellphone data also plays a major role in guilty pleas. Price said there were “countless” examples of defendants pleading guilty after losing motions to suppress cell-site data. “In situations where the government’s case centered on the cellphone data, this issue was the whole ballgame and meant the difference between going to trial or going to jail,” he said.

Some defendants convicted on cellphone data obtained without warrants have argued unsuccessfully that authorities violated their Fourth Amendment rights. That includes Quartavious Davis, sentenced to 162 years in federal prison for a string of Florida robberies, and Aaron Graham and Eric Jordan, convicted of partnering in a string of Baltimore stickups and sentenced to 147 years and 72 years in federal prison, respectively.

Cellphone data was also used as a key piece of evidence in the murder case against Adnan Syed, featured in the podcast “Serial.”

Drawing a line

While cellphone data has a variety of legitimate law enforcement uses, there’s also concern that it can be used to suppress dissent — making the Supreme Court’s ruling on Friday significant for free speech advocates.

In 2017, police arrested dozens of people who protested President Donald Trump’s inauguration. Rachel Levinson-Waldman, senior counsel at the NYU School of Law’s Brennan Center for Justice, said that in such a case, where prosecutors wanted to investigate how protesters organized or gathered, historical cellphone data could play a crucial role.

The Carpenter decision "is going to have pretty wide implications for the kinds of criminal cases where the government is looking at people’s participation in associative activities and political activity,” Levinson-Waldman said. “Now, there won’t be this time machine for location information.”

There are many other ways that the government can try to track people that don’t currently require a warrant, Akerman noted. But none of them are as invasive as tracking cellphone location.

“It’s automatic and involves every person in this country and it’s 24-7, basically,” Akerman said. “So the expectation of privacy (in Friday’s Supreme Court ruling) is really drawing a line on a gross example.”