The US Supreme Court has approved a rule change that could allow law enforcement to remotely search computers around the world.
Previously, magistrate judges could order searches only within the jurisdiction of their court, often limited to a few counties.
The US Department of Justice (DoJ) said the change was necessary to modernise the law for the digital age.
But digital rights groups say the move expands the FBI's hacking authority.
The DoJ wants judges to be able to issue remote search warrants for computers located anywhere that the United States claims jurisdiction, which could include other countries.
A remote search typically involves trying to access a suspect's computer over the internet to explore the data contained on it.
It has pushed for a change in the rules since 2013, arguing that criminals can mask their location and identity online making it difficult to determine which jurisdiction a computer is located in.
'Only mechanism available'
"Criminals now have ready access to sophisticated anonymising technologies to conceal their identity while they engage in crime over the internet," said DoJ spokesman Peter Carr.
"The use of remote searches is often the only mechanism available to law enforcement to identify and apprehend them.
"The amendment makes explicit that it does not change the traditional rules governing probable cause and notice."
It said the change would not give law enforcement any new authority not already permitted by law.
However, groups such as the American Civil Liberties Union (ACLU) have warned that the change could expand the FBI's ability to conduct mass hacks on computer networks.
'Millions of computers'
"Such a monumental change in the law should not be snuck by Congress under the guise of a procedural rule," said Neema Singh Guliani of the ACLU.
In 2015, search giant Google also opposed the change, which, it said, "threatens to undermine the privacy rights and computer security of internet users".
Oregon Senator Ron Wyden said the change had "significant consequences for Americans' privacy", and said he would seek to reverse the decision.
"Under the proposed rules, the government would now be able to obtain a single warrant to access and search thousands or millions of computers at once; and the vast majority of the affected computers would belong to the victims, not the perpetrators, of a cybercrime," he said in a statement.
Congress can still opt to reject or modify the changes to the federal rules of criminal procedure - but if it does not act by 1 December the change will take effect.