The state’s highest court for the first time on Tuesday extended the right to privacy to encompass real-time cellphone location data, but preserved the right of law enforcement to “ping” cellphones in emergencies, such as a search for an armed murder suspect.
In a unanimous decision, the Supreme Judicial Court held that state constitutional protections against unwarranted and excessive government intrusion into personal lives must keep pace with technological innovations.
Cellphones are “almost a feature of human anatomy,” with tracking technology that police can “ping” to learn a person’s current location without their knowledge, Justice Scott L. Kafker wrote for the court. “This extraordinarily powerful surveillance tool finds no analog in the traditional surveillance methods of law enforcement and therefore grants police unfettered access ‘to a category of information otherwise unknowable.’ ”
The ruling prohibits police from unilaterally demanding that phone companies turn over cell tower location information. In most circumstances, police will now need a search warrant to obtain the information, the court ruled.
“The power of such unauthorized surveillance . . . is too susceptible to being exercised arbitrarily by law enforcement — precisely the type of governmental conduct against which the framers sought to guard,’’ Kafker wrote.
Matthew R. Segal, the legal director for the American Civil Liberties Union Foundation of Massachusetts, hailed the ruling as a “truly important decision.”
“It reaches new territory because it’s holding that a single ping of a cellphone is a search that would normally require a warrant,” he said.
The practices of law enforcement agencies in the state, Segal said, “are a bit opaque.”
“Only they know how often they have been warrantlessly manipulating electronic devices to learn information,” he said.
The warrant requirement, Segal said, can be “satisfied by good police work.” He did not think the decision would be onerous for law enforcement.
In its decision, the SJC ruled law enforcement can still require cellphone companies to ping phones without a warrant if there are “exigent circumstances,” such as danger to the public or a threat that the suspect will flee or destroy evidence.
Karen Pita Loor, a Boston University law professor, also applauded the ruling, saying it “places limits on a police officer’s ability essentially to obtain data without probable cause.”
She said the ruling was “an expansion” of a Supreme Court decision from last year that found that the government generally needs a warrant to collect troves of location data about the customers of cellphone companies. The Fourth Amendment, she said, offers protections from government intrusion.
“Courts are having to grapple with how this applies to this new technology,” she said.
Courts in other states have wrestled with similar cases, according to Jennifer Lynch, the surveillance litigation director for the Electronic Frontier Foundation. She said in an e-mail that her group filed an amicus brief in a similar case before the Maine Supreme Court and is awaiting an opinion on that matter. In 2014, she said, the Florida Supreme Court held that warrantless, real-time tracking violated the Fourth Amendment.
Tuesday’s ruling came in the first-degree murder prosecution of Jerome Almonor, who was arrested by police just hours after he allegedly shot a man to death in Brockton in 2012.
Lonnie Robinson was killed while sitting in the passenger seat of a car parked in a driveway, the ruling stated. The driver identified Almonor as the man who fired the fatal shot with a sawed-off shotgun.
Police obtained Almonor’s cellphone number and learned that his former girlfriend lived at a house on Clarence Street. Police ordered the cellphone company to “ping” Almonor’s phone, and the data showed he was in one of a handful of houses on Clarence Street — including his former girlfriend’s home.
Police were allowed to search the house by the homeowner, and Almonor was found sleeping in an upstairs bedroom — where police also found a sawed-off shotgun and a bulletproof vest. He was arrested and charged with murdering Robinson.
Plymouth Superior Court Judge Cornelius J. Moriarty II ruled that tracking Almonor’s cellphone violated his privacy rights and that law enforcement needed a search warrant to obtain the cellphone data. The judge barred the Plymouth district attorney’s office from using the evidence against Almonor at trial.
The SJC reversed Moriarty’s decision, saying police were justified in proceeding without a warrant.
“We conclude that under the circumstances at the time the defendant’s cell phone was pinged, the police had reasonable grounds to believe that obtaining a warrant would be impracticable because taking the time to do so would have posed a significant risk that the suspect may flee, evidence may be destroyed, or the safety of the police or others may be endangered,’’ the court ruled.
In a statement Tuesday, Berkshire District Attorney Andrea Harrington called the ruling “a sound and proper balancing of individual privacy interests and public safety.”
“The exigent circumstances exception that the court allowed for in this particular type of search will be an effective tool in protecting the public from ongoing dangers, while ensuring individuals’ constitutionally protected expectation of privacy,” she said.
Cape and Islands District Attorney Michael O’Keefe said he hoped the ruling will change little in terms of law enforcement’s approach to investigations. He said the ruling “leaves room for the police to do their job,” and said police are “generally very, very careful that they are not intruding on people’s privacy rights.”
“Massachusetts has a particular zealous reputation for guarding individual privacy rights,” he said during a phone interview Tuesday.
Correction: Due to a reporting error, this story should have stated that the decision extended privacy protections to real-time cell phone location data for the first time. The SJC in 2014 first decided cell phone data was entitled to constitutional privacy protections.
Material from The New York Times was used in this report. John R. Ellement can be reached at ellement@
globe.com. Danny McDonald can be reached at email@example.com.