Rep. Trey Gowdy, chairman of the House Select Committee on Benghazi, blasted the State Department Monday for refusing to name the documents it has withheld under vague "confidentiality" exemptions more than five weeks after a deadline to produce a list.

"For nearly a year and a half, the State Department has withheld documents and information about Benghazi and Libya from the American people's elected representatives in Congress," Gowdy said. "Whatever the administration is hiding, its justifications for doing so are imaginary and appear to be invented for the sake of convenience."

The State Department had been ordered to hand over a list by May 18 of the documents it had refused to produce and explain the legal justification for withholding them.

But months after the select committee issued a subpoena for those records, State Department officials are still refusing to clarify what they are withholding and why.

"That's not how complying with a congressional subpoena works, and it's well past time the department stops stonewalling," Gowdy said.

The State Department and the White House have spent months frustrating committee investigators with slow and incomplete responses to requests for documents and access to witnesses.

As the committee prepares to publish its final report, members have acknowledged the difficulty in extracting information from the administration, a challenge that may leave gaps in the conclusions the committee is able to draw.

The select committee was the first to uncover Clinton's use of a private email server despite the several congressional investigations into Benghazi that preceded it.

Democrats have accused Gowdy's panel of pursuing a partisan agenda in an effort to weaken Hillary Clinton's presidential prospects.