The head of the congressional committee investigating Hillary Clinton’s handling of classified information on her private email server blasted the State Department on Wednesday for hindering its probe by not providing requested documents.

Rep. Jason Chaffetz, R-Utah, chairman of the House Committee on Oversight and Government Reform, also served notice on the White House that its credibility in claiming transparency is on the line.

“We need to understand the gravity of this. There is a reason the State Department, years after, is still holding off on giving us documents,” Chaffetz told Fox News. “The more they want to hold them, the more curious we are on what’s in them.”

Chaffetz said the State Department has been “dreadfully slow” in turning over some 30,000 additional documents, and that he expected the investigation to last well into 2018.

“Agencies like to brag about how many thousands of documents they’ve given us, but it’s not about the count — it’s about getting 100 percent of them.”

The committee head said his panel plans to investigate the case from “top to bottom” — including investigating the meeting on an airport tarmac between then-Attorney General Loretta Lynch and President Bill Clinton in “such close proximity” to the conclusion of the FBI’s investigation of the email scandal.

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According to Chaffetz, it could be as many as 300 people who mishandled classified information at the State Department.

“It’s hard to believe nobody is even going to get a slap on the hand — somebody, people need to be held accountable,” he said.

On Feb. 16 Chaffetz sent a letter, which committee Democrats called a “waste of time,” to the Justice Department requesting the prosecution of former State Department employee Brian Pagliano, who failed twice last September to comply with a committee subpoena.

“Apparently, Chairman Chaffetz and President Trump are the only two people in Washington today who think we should still be investigating Secretary Clinton,” Rep. Elijah Cummings, D-Md., said in a statement last month. “The Oversight Committee can’t afford to be distracted by political vendettas against Hillary Clinton while our constituents are begging us to conduct responsible oversight of President Trump.”

A Justice Department spokesperson told Fox News the letter is “under review.”

Separately, the State Department said it would continue to seek a dismissal of a lawsuit by Judicial Watch that seeks more Clinton emails from her tenure at the department.

“For the State Department and DOJ lawyers to still say their position is what the Obama administration’s position was is not good,” Tom Fitton, president of Judicial Watch, told Fox News. “The default position of the bureaucracy is secrecy, and in terms of partisanship, it’s to protect Mrs. Clinton.”

“I think the Obama White House and agencies were delaying this as much as they could, but there’s no excuse now — we’re going to find out just how transparent the Trump administration is going to be,” Chaffetz said. “There is no doubt — we’re not letting go of this now.”