Former FBI Director James Comey is scheduled to testify before the Senate Judiciary Committee later this month, Chairman Lindsey Graham announced Thursday.

Graham told Fox News Comey had agreed to appear before the panel at the end of the month as part of his committee's inquiry into the FBI's 2016 Russia investigation. Graham said former special counsel Robert Mueller had declined an invitation to appear.

“I talked to Mr. Mueller. He felt like he didn’t have enough time to prepare. And I will honor that request,” Graham said before a Judiciary Committee meeting.

Comey’s scheduled Sept. 30 testimony is to be public. Graham characterized Comey's testimony as significant for the review of the FBI investigation. President Trump repeatedly called the investigation a politically motivated "witch hunt," but a bipartisan report issued last month by the Senate Intelligence Committee outlined the extent of cooperation between the campaign and Russia.

Comey agreed to testify without a subpoena, said Graham, who added his staff is having trouble securing testimony from former FBI Deputy Director Andrew McCabe. 

Mueller’s decision against appearing before the committee, as Graham described it, could complicate the Senate GOP-led investigation, which Trump has openly encouraged. Last week, the Justice Department released documents suggesting top Mueller deputies erased information from at least 15 phones, citing forgotten passwords, physical damage and missing hardware, Politico reported. 

The committee already has heard from several witnesses in front of the cameras and in private, including former Deputy Attorney General Rod Rosenstein and former Acting Attorney General Sally Yates.

Mueller's investigation concluded it could not prove a conspiracy between the Trump campaign and Russia, but outlined multiple contacts between campaign officials and Russian entities.