Should police need a warrant to access CharlieCard data?

Every time riders tap their plastic CharlieCards at a subway station or on a bus, a new piece of information flows into a central database at the Massachusetts Bay Transportation Authority, linking the card’s 10-digit serial number with the time and location it was used.

Now, a case before the state’s highest court — as well as a push from state lawmakers — could limit access to that electronic trail by law enforcement.

The Supreme Judicial Court is considering whether police need a search warrant to check CharlieCard data logs as part of an appeal of the 2017 murder conviction of Josiah Zachery, who was found guilty of killing a rival gang member in Boston in 2015 near Forest Hills.

When he was arrested, Zachery had a CharlieCard on him, which police used to determine his location earlier on the day of the murder, with the help of the MBTA. Using CharlieCard data that showed where Zachery entered the subway system, police searched for him on the T’s security camera footage, matching the clothes he was wearing with witness descriptions of the shooter.

Zachery’s lawyer, Jennifer O’Brien, argued the search of MBTA information was illegal because police did not obtain a warrant from a judge. Without a warrant to investigate the CharlieCard, police should not even have examined it closely enough that they could determine its small-print serial number, she said.

“We’re arguing that the defendant had an expectation of privacy for his CharlieCard,” she said.

And, O’Brien added, unauthorized searches for subway entry data are particularly unfair to low-income, minority residents who rely more on public transit; Zachery, 24, is Black.

Warrantless searches are “more invasive to people for who this is their only means of transportation,” she said.

In court filings, prosecutors have argued that MBTA riders have no expectation of privacy on the public transit system, in part because “the information that a commuter provides to the MBTA by using a CharlieCard at a fare gate is precisely what the user intends to communicate — that they have paid for a public transportation trip at a particular location.”

Suffolk District Attorney Rachael Rollins, whose office is arguing the case for the state, declined further comment.

The SJC case is the latest gust in a whirlwind of debate about privacy in an age of increasing surveillance, particularly when people are traveling, featuring thorny questions about police access to cellphone information and roadway systems that automatically record license plate images.

“As technology changes, the court’s interpretation of the Constitution needs to adapt to maintain a balance between our rights to privacy and the police’s ability to track us any day and time,” said Matthew Spurlock, an appeals attorney at the Committee for Public Counsel Services who is filing a brief in the CharlieCard case arguing a warrant should be required.

There have been several recent court decisions at the state and federal level about location tracking that have produced somewhat divergent findings.

The SJC and the US Supreme Court have both ruled police need a warrant to search for cellphone location data since people rarely go anywhere without their phones, including private locations.

This spring, the SJC ruled that police were within their rights to log a suspect’s movement over the Cape Cod bridges using automatic license plate readers, because the cameras in question were in a limited and highly public location. However, the court signaled that it was uneasy with the idea of more widespread monitoring and said that tracking a motorist across many areas or in more private locations could be deemed unconstitutional.

Those precedents set up several possible directions for the court’s thinking on CharlieCards, said Michael Cassidy, a Boston College criminal law professor.

“The question, I think, for this CharlieCard case is whether it’s more like cellphone location data or more like traveling over the Sagamore Bridge,” he said. “It’s more like traveling over the Sagamore Bridge in one sense: if you go out on a public subway, people can see where you are or where you’re going.”

But because CharlieCards can also provide information about travel patterns across the entire subway system, the court may look at them as more similar to cellphones, he added.

“It’s much more invasive” than cameras on the bridge, he said.

Rollins’s office has argued in court papers that CharlieCard data is not overly invasive because riders only tap their cards at the start of the ride. MBTA police have access to the database and can provide information to law enforcement upon request, prosecutors said.

The MBTA and its Transit Police department declined to comment. The agency’s privacy policy notes that in “certain circumstances, the MBTA may also share your personal information with the police and other law enforcement agencies for the purposes of the prevention or detection of crime.”

Moreover, CharlieCards only contain serial numbers, and generally do not identify the card holder — unless riders have specifically linked them to an online payment account. Even those ordered through a corporate pass program are usually tied to the employer, not the specific employee.

In the Zachery case, his card was taken during his arrest, along with other belongings, after he was found in the area of the shooting, and used to match his movement to video evidence.

“It shows how the police can aggregate data from different sources to create a much fuller picture of what is happening,” said Jennifer Lynch, an attorney at the Electronic Frontier Foundation, a digital civil rights group that believes tracking on public transit should require a warrant. “It also shows how confusing the state of the law is right now and how difficult it is for law enforcement officials to be sure they’re doing the right thing. The easiest way to approach it is to just say you need a warrant no matter what.”

The court case comes as the MBTA adopts a new all-electronic payment system that could increase the amount of personal identifying information attached to CharlieCards. It would also allow the MBTA to set up a system where riders use their card to enter and exit the system, creating more exact location data.

In anticipation of that switch, the Massachusetts Senate included a requirement that police must get a warrant to obtain fare data in a transportation funding bill that passed this month.

Kade Crockford, a privacy specialist at the American Civil Liberties Union of Massachusetts, said the coming fare system makes it more urgent that courts or the Legislature add the warrant protections now.

“The MBTA’s plan to move to a cashless fare system opens up new questions about the technology,” Crockford said. “The stakes are raised.”