Washington (CNN) CNN is suing the FBI for memos documenting what about 500 witnesses told special counsel Robert Mueller and the FBI during the Mueller investigation.

The memos, called 302s, form the backbone of the Mueller report in many cases.

They've become a crucial piece of what various groups, from news organizations to the House of Representatives, want to see from the Mueller investigation.

The memos document what key cooperators -- like former deputy campaign chairman Rick Gates, former national security adviser Michael Flynn, Trump's former personal lawyer Michael Cohen and former White House counsel Don McGahn -- told Mueller's team, according to citations in the Mueller report.

CNN went to the courts because the FBI hadn't fulfilled a records request from March for the memos, as required under the Freedom of Information Act, which provides the public access to government records with some exceptions, according to the complaint.

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