FBI's use of National Security Agency (NSA) data has been contentious ever since the details of mass surveillance were revealed by its contractor Edward Snowden in 2013.
A Foreign Intelligence Surveillance Court has now ruled that FBI officials "improperly" searched through NSA records.
This ruling puts a shadow on how American intelligence agencies collect surveillance data.
It includes both upstream and downstream data and has been collected in the past under section 702 of the Foreign Intelligent Surveillance Act.
The court found thousands of searches conducted between 2017 and 2018.
Improper searches are in direct violation of the Fourth Amendment of the US Constitution.