Washington (CNN) Thousands of pages of internal FBI documents related to the bureau's decisions emanating from the 2016 election were delivered to congressional investigators Monday, officials said -- the latest document handover in a six-month-old probe that has proved frustrating for Republicans up to President Donald Trump.

The delivery comes amid a chorus of complaints that the Justice Department and FBI are working too slowly to produce the documents for the House Judiciary and Oversight committees. The Justice Department Monday also said it was designating a sitting US attorney to oversee the process.

"What does the Department of Justice and FBI have to hide?" Trump tweeted on Saturday. "Slow walking - what is going on? BAD!"

The joint congressional probe, announced in October, is examining the moves made by the FBI as it investigated Hillary Clinton's private email server.

Late last month, House Judiciary Chairman Bob Goodlatte subpoenaed the Justice Department, criticizing the agency for its responsiveness to the probe and expanding the inquiry to include questions of surveillance abuse and the decision to fire former FBI deputy director Andrew McCabe.

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