The chairman of a special House committee investigating the deadly 2012 attacks in Benghazi, Libya, promised Tuesday to “ratchet up” an inquiry he said is being hobbled by resistance from the Obama administration.

Rep. Trey Gowdy, R-S.C., said the committee has recently received 15,000 pages of new documents, but needs greater cooperation from the State Department and other agencies to do its job.

“Letters haven’t worked. Southern politeness hasn’t worked. We’re going to ratchet it up,” Gowdy said at the end of a tense, two-hour hearing.

Rep. Elijah Cummings of Maryland, the panel’s senior Democrat, complained that the committee’s investigation was moving at a “glacial” pace and said he and other Democrats “have grave concerns about the partisan path this committee has taken over the past year.”

Cummings accused Gowdy of excluding Democrats from the investigation, noting that the chairman has held private meetings with at least five witnesses without Democrats present.

“You may have authority under House rules to conduct secret interviews and exclude Democrats, but doing so forfeits your right to continue calling this investigation ‘bipartisan,’ ” Cummings told Gowdy.

The remarks came as the committee held its third public hearing since its creation last May amid promises of bipartisanship and cooperation. Eight months later, the panel has devolved into finger pointing and accusations of political grandstanding and power plays.

Rep. Adam Schiff, D-Calif., said Gowdy and other Republicans had placed the committee’s credibility at risk by excluding Democrats from key parts of the investigation. The GOP-led panel appears to be conducting “a drawn-out, partisan fishing expedition” to discredit prominent Democrats, Schiff said.

Chief among the possible targets is former Secretary of State Hillary Rodham Clinton, who led the State Department when the attacks occurred and is widely expected to run for president in 2016. Gowdy said after the meeting that he was likely to call Clinton as a witness, but not until he receives more emails and other documents from the State Department.

The 12-member panel was created last May to investigate the September 2012 attacks on a U.S. post in eastern Libya that killed Chris Stevens, the U.S. ambassador, and three other Americans.

Schiff and other Democrats complained that Republicans waited for nearly six months to request documents from the State Department and other agencies. Schiff called on Gowdy to lay out the scope of his investigation immediately and adopt a set of rules “that will give Congress and the country the assurance that this will not be yet another politicized and partisan exercise at taxpayer expense.”

If the investigation “does not produce a bipartisan report, it will have been a complete failure,” Schiff said.

Rep. Mike Pompeo, R-Kan., said Democrats were being hypocritical, complaining about the panel’s slow pace while not helping it move more quickly.

“They talk about us being too slow, yet they act as if their job is to play defense” for the Obama administration, Pompeo said.

The partisan tone marked a sharp turnaround for a panel that had won praise for a bipartisan approach through its first two public hearings. At one point, Gowdy allowed a Democratic member to participate by video conference after he underwent surgery.

Gowdy’s approach has drawn criticism from some conservatives, who accuse him of failing to stand up to what they see as obstruction by the Obama administration related to the events in Benghazi, a topic that’s been the subject of numerous congressional investigations.

A report by the House Intelligence Committee report last fall found that the CIA and the military acted properly in responding to the 2012 attacks. Debunking a series of persistent allegations hinting at dark conspiracies, the panel determined there was no intelligence failure, no delay in sending a CIA rescue team, no missed opportunity for a military rescue and no evidence the CIA was covertly shipping arms from Libya to Syria.