Last Updated Oct 18, 2015 1:27 PM EDT
Maryland Rep. Elijah Cummings, the top Democrat on the House committee investigating the 2012 attacks on a U.S. diplomatic facility in Benghazi, Libya, said former Secretary of State Hillary Clinton's upcoming testimony represents a "sad day" for those investigating the attacks.
"I think it's a sad day for all of us because we made a commitment to the families. The families came in with tears in their eyes literally and said, 'Please do not make this a political football.' That's exactly what's happened," Cummings sad in an interview with CBS' "Face the Nation" Sunday. "They said, 'Find out more information about what did happen,' and then they asked us to do one other thing and that is try to make sure you figure out how this does not happen again and I think we failed at all three."
In a separate interview on "Face the Nation," Chairman Trey Gowdy, R-South Carolina denied that the committee's work is focused on Clinton and said his GOP colleagues who say it is should "shut up." The committee has uncovered new information about what happened before, during and after the attacks, he said.
But Cummings, who appeared after Gowdy, said he is "trying to shift back to where we should have been all along."
"He keeps saying, 'Don't listen to what they said, they don't know anything.' Well, we were on the committee, too," Cummings said. "We know what's been going on. And listen to this: He has not yet interviewed the head of the C.I.A., but he brought in [former Clinton aide Huma] Abedin. He has not yet interviewed the head of Joint Chiefs, the Secretary of Defense, none of that."
"When he talks about these 50 witnesses, we still have been zeroed in on Hillary Clinton and there is absolutely no doubt about that and it's very unfortunate," Cummings said.
He has called on Gowdy to release the transcripts of all the people who have been interviewed by the committee and said the Democrats will begin doing so this week.
"All the things that Mr. Gowdy and others have been accusing Hillary Clinton of over the last three years, not a witness verifies any of that," Cummings said. Clinton "was not involved in the decision with regard to the security on the ground there and it also addresses the issue of whether she was responsible for this illicit gun running between Libya and Syria. And then, the big allegation that they've made is that she ordered a stand down of folks to help our four diplomats. That is absolutely not true."
Cummings called Gowdy a "good man" who has been "under pressure for a lot on the right." But he also said his Republican counterpart tossed out a plan he presented to the Democrats for carrying out the investigation, and instead "went straight for Hillary Clinton." He faulted Gowdy for having not called a hearing since January.
Asked whether Clinton bears some responsibility for the deaths of the four Americans during the attack, Cummings said, "Of course she does and she says she does, she's taken full responsibility. And when she comes in, I want her to tell us how we can best protect our diplomatic core and our embassies. And I wanted her to tell us exactly what we can do as a Congress to address these issues also."
Cummings penned a letter early Sunday morning to the Benghazi committee chair refuting Gowdy's accusations that classified information had passed through Clinton's private email account. Contrary to Gowdy's earlier claim that Clinton's emails contained "some of the most protected information in our intelligence community," Cummings said, the CIA did not deem the details to be classified. The messages in question were sent by Sidney Blumenthal, a longtime Clinton family confidant and former Clinton Foundation adviser.
Cummings pointed to the allegations as proof that Gowdy's partisanship brought "discredit on this investigation." He also demanded the chairman issue "an immediate apology" to former Secretary Clinton.
Gowdy gave a swift response to Cummings shortly after his "Face the Nation" appearance. In his own letter, Gowdy claimed that "the nature of the information provided to the Secretary" by Blumenthal was "highly relevant" to the investigation.
The South Carolina Republican also disputed that the CIA ever informed the committee that his accusations were wrong.
"My understanding is the CIA advised the Committee in a very brief email late Saturday night that it had reviewed the material in question and asked for no material to be redacted," Gowdy wrote. "And in fact, additional information remains in the document that ordinarily would be considered highly sensitive."
CBS News' Reena Flores contributed to this story.