The decision by the Foreign Intelligence Surveillance Court was released by the Office of the Director of National Intelligence (ODNI), Reuters reported.
The searches occurred in the course of US crime investigations including the Jan. 6 Capitol riots and protests after the 2020 killing of George Floyd, the court said.
The intelligence database stores digital and other information on individuals. The Foreign Intelligence Surveillance Act allows the FBI to search without a warrant communications of foreigners abroad including their conversations with Americans.
The court ruling found the FBI violated rules around the use of the database, created under Section 702 of the FISA Act with its searches.
Specifically, the court found that searches as part of probes into crimes between 2016 and 2020 violated the rules because there was "no reasonable basis to expect they would return foreign intelligence or evidence of crime", although the FBI believed this was "reasonably likely," the decision said.
The revelations came as US President Joe Biden's administration is trying to garner congressional support to keep surveillance powers under Section 702, which is set to expire later this year.
The ODNI said the FBI tightened its procedures in mid-2021 and 2022. "As a result, these compliance incidents do not reflect FBI’s querying practices subsequent to the full deployment of the remedial measures," the office said.
An FBI spokeswoman did not immediately respond to a request for comment.