One of President Donald Trump’s closest confidantes is speaking privately next week to congressional investigators looking into Russia meddling, according to multiple sources familiar with the matter. Keith Schiller, a long-time adviser to Trump who left the White House in September after serving as director of Oval Office Operations, is expected to appear next Tuesday before the House intelligence committee, according to two of the sources. Schiller had long been by the President’s side, and was the aide dispatched by Trump in May to deliver the news to the FBI that James Comey had been fired as bureau director. The Russia interviews – at least a half-dozen this week and next – show that the committee’s probe, which has often been marred by partisan squabbles, is still chugging along through its witness list. The witnesses coming before the panel include Trump administration and campaign officials, former Obama administration officials and one of the eight individuals who was present for the June 2016 Trump Tower meeting between Donald Trump Jr., a Russian lawyer and several others, according to the sources. Rep. Mike Conaway, the Texas Republican who is leading the probe after House intelligence chairman Devin Nunes stepped aside earlier this year, has said he wants to wrap up the inquiry as quickly as is feasible. But California Rep. Adam Schiff, the top Democrat on the panel, has accused Republicans of trying to rush certain witnesses so the investigation can end. The charges unveiled this week from special counsel Robert Mueller’s investigation add another complicating factor for the congressional Russia investigations, and will make it tougher for the House panel to interview Paul Manafort, Rick Gates and George Papadopoulos, who were all on the committee’s witness list. Schiller, however, has been of high interest to Democrats on the panel given his close ties to Trump and his inner circle. He could not be reached for comment. But the committee has kept a busy schedule. On Thursday, the committee is scheduled to have two witnesses appearing: former Trump campaign foreign policy adviser Carter Page and Ike Kaveladze, one of the eight people in the Trump Tower meeting. Page is testifying at a hearing the committee has called “open in a closed space,” which means it will be held behind closed doors, but a transcript will be released three days afterward. Kaveladze, who worked for the Russian oligarch who sought to broker the meeting between the Russian lawyer, Natalia Veselnitskaya, and Trump Jr., will meet with investigators behind closed doors. A lawyer for Kaveladze declined to comment to CNN for this story. Kaveladze was asked to go to the meeting to act as Veselnitskaya’s translator, though she ended up bringing a translator with her. On Wednesday, the committee interviewed Mary McCord, the former acting assistant attorney general at the National Security division, multiple sources said. McCord, who left the Justice Department in April, was one of the top Justice Department officials overseeing the FBI’s Trump-Russia probe. McCord declined to comment on her testimony. The committee is also expected to interview another former Justice Department official on Friday: former acting Attorney General Sally Yates, according to a source. Yates was previously invited to testify before the House intelligence committee in the spring, and she did appear before the Senate judiciary subcommittee in May. Yates testified that she had warned the White House that then-national security adviser Michael Flynn was at risk of blackmail by the Russians. A spokesperson for Yates could not be reached for comment. And next week, Glenn Simpson, the co-founder of Fusion GPS – the intelligence firm that paid British intelligence officer Christopher Steele to compile an opposition research dossier on Trump – is expected to appear before the House panel after he was subpoenaed. A spokesperson for Fusion GPS did not respond to an inquiry seeking comment. Last month, two Fusion GPS partners invoked their constitutional rights not to answer questions before the committee. Since they appeared, a settlement was reached between the House panel and Fusion GPS over the intelligence firm’s bank records, and the committee said it would obtain the records relevant to its investigation. Nunes has been pushing hard to pressure Fusion GPS to reveal more about its efforts, and has demanded to know the extent that the Justice Department relied on the Steele dossier to launch its Russia investigation last year. In recent days, the funders of Fusion GPS’ opposition research on Trump have also come out: The law firm Perkins Coie said it paid Fusion GPS on behalf of the Hillary Clinton presidential campaign and the Democratic National Committee, and last week the conservative news outlet the Washington Free Beacon said it paid for opposition research on Trump during the Republican primary.