Law enforcement officials investigating the January 6, 2021, rioting at the U.S. Capitol mounted the largest electronic dragnet in American history, scooping up the location data of thousands of smartphone users who were in and around the building that day in their efforts to prosecute perpetrators of the violence and vandalism that occurred.

Court records unsealed in the case of one of the 900 defendants charged in connection with the events of that day, David Rhine, show that the FBI sought and received a “geofence warrant” forcing Google to turn over the location data of more than 5,700 smartphones present at the Capitol that day. Lawyers for the defendant are alleging that the warrants amount to an unconstitutional violation of the Fourth Amendment’s protection from unreasonable searches and seizures.

In a motion to suppress evidence obtained from Google being utilized to prosecute their client, Mr. Rhine’s lawyers argued that the FBI used the warrant to unlawfully identify individuals who were “engaged in personal and protected activities (such as exercising their rights under the First Amendment)” without the requisite probable cause.

“The government enlisted Google to search untold millions of unknown accounts in a massive fishing expedition,” one of Mr. Rhine’s public defenders, Rebecca Fish, wrote in the motion. “The warrant forced Google to act as an adjunct detective, scouring the account of ‘numerous tens of millions’ to generate a lead for the government.”

Ms. Fish’s motion was first reported by the independent journalist Marcy Wheeler on her Empty Wheel blog.

The legal questions raised by the increasing use of geofence warrants by law enforcement — Google says it now receives some 10,000 requests a year related to such warrants — are new, but in the few instances in which constitutional questions have been raised judges have more often than not sided with defendants and ruled that the warrants are illegal. A decision in Mr. Rhine’s case is pending.

Several courts have ruled that evidence obtained using geofence warrants is inadmissible, but the issue is far from settled. The Supreme Court ruled in 2017 in Carpenter v. U.S. that Americans enjoy Fourth Amendment protections against the unreasonable search of data that they do not own — such as data gathered and stored by the likes of Google — but still the practice persists. A subsequent case, U.S. v. Chatrie, also ruled the warrants unconstitutional, but the judge said the evidence was admissible anyway because the government acted in good faith in obtaining the warrant.

Even the technology companies charged with responding to the warrants are increasingly uneasy about their proliferation. A group of them, including Google, Microsoft, and Yahoo, came out in support of a bill now pending in the New York state legislature that would be the first in the nation to ban both geofence warrants and similar warrants seeking information about suspects’ internet keyword searches.

“This bill, if passed into law, would be the first of its kind to address the increasing use of law enforcement requests that, instead of relying on individual suspicion, request data pertaining to individuals who may have been in a specific vicinity or used a certain search term,” the industry group, called Reform Government Surveillance, said in a two-sentence statement in support of the bill.

The technology used by the FBI is similar to what Chinese authorities have been using to track people who attended recent protests over the country’s strict anti-Covid lockdowns. According to a report in the Wall Street Journal, residents of Shanghai, Beijing, and other cities have sought legal advice after being contacted by police asking about their whereabouts during the protests.

One university student at Beijing who attended a protest over the weekend told lawyers that police contacted the school he attends to inquire about his activities. The school, he said, told him that police had traced him to the protest using location data from his mobile phone. He was asked by authorities to submit a written declaration explaining why he was in the vicinity of the protests.

Authorities in China, however, are going beyond the location data available to the FBI and U.S. authorities, reportedly using apps ostensibly designed to monitor Covid exposure to track protesters’ movements. Because U.S. law is so fuzzy on the issue, the possibility that American law enforcement could do something similar and use any number of apps on Americans’ smartphones — from fitness trackers to traffic maps to ride-sharing apps — to provide location and other data to law enforcement is not out of the realm of possibility.