Several of the nine justices across the ideological spectrum made comments indicating that the absence of a court-issued warrant is troubling. The court could potentially rule that such a practice amounts to an unreasonable search and seizure under the U.S. Constitution’s Fourth Amendment.

The justices heard an extended 80-minute argument in an appeal brought by a man named Timothy Carpenter convicted in a series of armed robberies in Ohio and Michigan with the help of past cellphone location data that linked him to the crime locations. His American Civil Liberties Union lawyers have argued that without a warrant such data amounts to a Fourth Amendment violation.

Law enforcement authorities routinely request and receive this data from wireless providers during criminal investigations as they try to link a suspect to a crime.

The legal fight has raised questions about the degree to which companies protect their customers’ privacy rights. The big four wireless carriers, Verizon Communications (VZ), AT&T (T), T-Mobile U.S. (TMUS), and Sprint (S), receive tens of thousands of these requests annually from law enforcement.

Verizon was the only one of those four companies to tell the Supreme Court that it favors strong privacy protections for its customers, with the other three sitting on the sidelines.

Police helped establish that Carpenter was near the scene of the robberies of Radio Shack and T-Mobile stores by securing from his cellphone carrier his past “cell site location information,” which tracks cellphone towers that relay calls.

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There is increasing scrutiny of the surveillance practices of U.S. law enforcement and intelligence agencies amid civil liberties concerns among lawmakers across the political spectrum.

The Supreme Court twice in recent years has ruled on major cases concerning how criminal law applies to new technology, both times ruling against law enforcement. In 2012, the court decided that a warrant is needed to place a GPS tracking device on a vehicle. In 2014, the court ruled that a warrant is required to search a cellphone seized during an arrest.

The ACLU has said police need “probable cause,” and therefore a warrant, in order to avoid a Fourth Amendment violation. But the U.S. Justice Department said probable cause should not be needed to obtain customer records under a 1986 federal law called the Stored Communications Act.

The Justice Department said prosecutors must show only that there are “reasonable grounds” for records that are “relevant and material” to an investigation to be provided.

Carpenter’s bid to suppress the evidence failed and he was convicted of six robbery counts. On appeal, the Cincinnati-based 6th U.S. Circuit Court of Appeals upheld his convictions, finding that no warrant was required for the cellphone data.

According to trial testimony, Carpenter organized most of the robberies and often supplied guns used in them. Witnesses said he usually would sit in a stolen car across the street from the targeted store and his accomplices would enter the store upon his signal, brandish guns, usher customers and workers to the back and direct employees to hand over the store’s new smartphones.

A ruling is due by the end of June.