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.

The new complaint was filed in DC District Court on Tuesday and will be handled by Judge Royce Lamberth.

Several news organizations and transparency groups are already pursuing documents from the Justice Department and FBI related to the Mueller investigation, and their cases are before other judges.

At least one case, brought by Buzzfeed News last month, also asks for the special counsel's office's 302s. Another group, the Electronic Privacy Information Center, is seeking a vast collection of records from the Mueller investigation in court.

CNN's initial request to the FBI asked for "FBI memoranda from any and all" of the witness interviews and the case files related to those witnesses.

Mueller and the Justice Department announced publicly in March that his team interviewed about 500 witnesses for the investigation. In describing many of the facts of the report, Mueller cites the memos from those interviews. As Mueller describes Russian attempts to interfere in the 2016 presidential election and attempts by the President to obstruct justice, he relies on what Trump associates and others told prosecutors and the FBI. (Mueller also cites emails and grand jury proceedings, among other evidence, but these documents aren't part of CNN's lawsuit.)

So far, only a few memos related to the Mueller investigation have been made public. Those came in the criminal case against Flynn as a judge prepared for his sentencing. The Washington Post recently asked a court for the removal of redactions in the 302 memo from Flynn's January 2017 interview with the FBI, in which he lied.

Separately, the House Judiciary Committee has asked the Justice Department and the White House for access to the memos from Mueller's interviews with 33 witnesses, including former Attorney General Jeff Sessions and with Jared Kushner.

The memos are also part of the subpoena the House Judiciary sent to Attorney General William Barr in April. At that time, the Democratic-led committee demanded access to the complete, unredacted Mueller report and "all documents" referenced in it or collected by Mueller. The committee approved holding Barr in contempt for failing to turn over the records. A full House vote is set for June 11 to hold Barr in contempt.