The big picture: Online privacy has long been a major cause for concern, with most of the allegations of shady practices typically leveled against large tech firms like Google and Meta. However, police and other government agencies have also come under fire for resorting to unethical - or at times outright illegal - means to obtain data.

The FBI recently admitted it purchased the location data of US citizens without obtaining a warrant. The acknowledgement came from FBI Director Christopher Wray at a Senate Intelligence Committee hearing on global threats.

Answering a question from Senator Ron Wyden of Oregon about whether the FBI purchases the phone-geolocation information of US citizens, Wray said the agency had previously done so for an unnamed national security project, but does not indulge in such practices any more.

According to Wray, "To my knowledge, we do not currently purchase commercial database information that includes location data derived from Internet advertising...I understand that we previously---as in the past---purchased some such information for a specific national security pilot project. But that's not been active for some time."

Instead of buying the location data of US citizens, Wray said the agency currently uses a "court-authorized process" to get data required for investigation. Wray did not specify whether the process involves getting a warrant or resorting to other legal means.

Wray's revelations mark the first time the FBI has admitted ever buying people's location data, despite persistent complaints from privacy activists and civil rights organizations about the policy.

As reported by Wired, a US Supreme Court ruling in the landmark 'Carpenter v. United States' case held that government agencies accessing the location data of US citizens without a warrant is a violation of the Fourth Amendment.

However, the ruling left a loophole that has since been exploited by many federal agencies, including US Customs and Border Protection (CBP) and the Defense Intelligence Agency (DIA). The Department of Homeland Security is also known to have purchased the location data of US citizens from private marketing firms in the past.

As expected, the revelations have set alarm bells ringing among privacy advocates and national security reform activists, who say such actions by the FBI and other investigative agencies could have dangerous consequences for the freedom and digital privacy of US citizens.

In a statement to Ars Technica following Wray's testimony, Electronic Frontier Foundation (EFF) Senior Staff Attorney Adam Schwartz said, "US government agencies must not be allowed to do an end run around the Fourth Amendment by buying private information from data brokers who collect information about the precise movements of hundreds of millions of people without their knowledge or meaningful consent.

"This extremely sensitive information can reveal where we live and work, who we associate with, and where we worship, protest, and seek medical care," he added.

Sean Vitka, a policy attorney at civil liberties and transparency advocacy group Demand Progress, termed the FBI's actions "horrifying" and said, "The public needs to know who gave the go-ahead for this purchase, why, and what other agencies have done or are trying to do the same." He also said Congress should formulate legislation to ban the practice entirely.