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The United States government has reportedly been issuing secret "keyword warrants" to Google for those who look up certain phrases and names, triggering worry over privacy in addition to concern for innocent internet users, who could unknowingly and erroneously become involved in major criminal investigations.

The controversial practice, used in secret to search for criminals for years, was revealed on Tuesday in "accidentally unsealed" court documents cited by Forbes. So far only a few such warrants have been made public, the most recent one stemming from a 2019 investigation in Wisconsin, where authorities had been searching for men accused of participating in the abuse and sexual trafficking of a minor.

In a bid to hunt down the criminals, officials asked that Google give up information—including IP addresses, account names and CookieIDs—for users who searched for the victim's name, two spellings of her mother's name and her address during a specific 16 days the same year she disappeared.

The tech giant did provide authorities with the data, but it's not clear how many names were turned over.

The investigation is still active and the warrant was among the unsealed documents obtained by Forbes. The Department of Justice had not even been made aware it was public until they were reached for comment, which they declined to provide.

Only two other "keyword warrants" had been disclosed prior to the Forbes article.

In 2020, officials again turned to Google, requesting information on anyone looking up the the address of an arson victim, who was also a witness in the government's racketeering case against singer R. Kelly. Another, detailed in 2017, revealed that a Minnesota judge signed off on a warrant asking Google to provide information on anyone who searched a fraud victim's name from within the city of Edina, where the crime took place.

Forbes was also able to identify a third unreported warrant filed in the Northern District of California in December of last year, which remains sealed.

"Trawling through Google's search history database enables police to identify people merely based on what they might have been thinking about, for whatever reason, at some point in the past," Jennifer Granick, surveillance and cybersecurity counsel at the American Civil Liberties Union, told Forbes.

"This never-before-possible technique threatens First Amendment interests and will inevitably sweep up innocent people, especially if the keyword terms are not unique and the time frame not precise. To make matters worse, police are currently doing this in secret, which insulates the practice from and regulation," she added.

In wake of the Forbes report, Jennifer Lynch, surveillance litigation director at the Electronic Frontier Foundation, identified another three warrants issued during used the investigation into serial Austin bombings in 2018, which resulted in the deaths of two people. They requested Google account information for individuals who researched multiple different addresses and some terms associated with bomb making, such as "low explosives" and "pipe bomb."

Similar orders were served on Microsoft and Yahoo for their respective search engines.

As for what data the gave to investigators, that information remains under seal.