Gizmodo reports nonprofit watchdog group American Oversight is suing the US Justice Department and five other agencies.
The lawsuit concerns the deployment of federal police against protesters in Portland and other cities to counter Black Lives Matter protests.
It accuses government officials of unlawfully withholding public records under the Freedom of Information Act.
Some of the records sought include legal guidance given to federal agents who have hidden their badges or refused to identify their agencies.
Encounters with the unidentified forces have seen them firing chemical agents and harmful projectiles at demonstrators, sometimes resulting in serious injury.