The chairman of the House select committee investigating the deadly 2012 attack on the U.S. Consulate in Benghazi, Libya defended himself against criticism from Democrats that he had excluded them from meetings with witnesses. 

In a strongly worded letter, Rep. Elijah Cummings, D-Md., claimed that Rep. Trey Gowdy, R-S.C., has used different standards for Republicans and Democrats and has held secret meetings with witnesses from the State Department and other agencies.

"Perhaps most importantly," Cummings wrote in a Jan. 23 letter, Gowdy has "withheld or downplayed information when it undermines the allegations we are investigating." The Associated Press obtained a copy of the letter, which comes before the panel is set to hold its third public hearing Tuesday.

Late Monday, Gowdy responded that he had the authority to unilaterally subpoena witnesses, but but he promised to give Democrats a week's notice before issuing such a subpoena.

"Bipartisanship is a two-way street," Gowdy said in a letter to Cummings. "I have known you to be a fair partner and expect for that cooperation to continue."

"No congressional select committee has ever had a requirement that sources meet with both sides at the same time, and the Benghazi Committee is no exception," committee spokesman Jamal Ware said in a statement. "Further, that the Democrats have released correspondence that attempts to politically characterize sources’ private discussions with the committee without proper context goes to the exact heart of why the Chairman will not require sources to talk to both sides."

Gowdy has said he will pursue the facts of the Sept. 11, 2012, attack on a U.S. post in eastern Libya that killed Chris Stevens, the U.S. ambassador, and three other Americans.

"Facts are neither Republican for Democrat," he said when the panel was created last May.

Gowdy's approach has drawn criticism from some conservatives, who accuse him of failing to stand up to what they see as resistance from the Obama administration to produce documents and witnesses related to the events in Benghazi, a topic that has 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.

Cummings, who has clashed with Republicans such as Rep. Darrell Issa, R-Calif., over Benghazi and other issues, has previously praised Gowdy for a bipartisan approach to the Benghazi inquiry.

Cummings's letter claimed that he and other Democrats "simply ask for a public debate and a vote by committee members on these actions when there is significant disagreement," Cummings wrote.

The Jan. 23 letter is the third Democrats have sent to Gowdy since November. None of the letters had previously been made public.

In one letter, dated Nov. 24, Cummings told Gowdy the committee inquiry has "taken a sharp turn for the worse and is becoming what you strenuously insisted it would not -- another partisan investigation of the Benghazi attacks that blocks Democrats from meaningful participation."

Fox News' Mike Emanuel and the Associated Press contributed to this report.