The Pentagon, however, said Monday it has kept Congress informed of the operation. Military investigators are looking into the exact circumstances of the October 4 raid, including how Sgt. La David Johnson was separated from the 12-member team as it was ambushed by 50 ISIS fighters.
"I did not," Sen. Bob Casey, D-Pennsylvania, responded to CNN's Chris Cuomo on "New Day" Monday whether he knew there were troops in Niger. "When you consider what happened here, the four sergeants lost their lives, I think there's a lot of work that both parties and both branches of government need to do. Not only to stay more informed but to focus on why we're there and what happened to get to the bottom of this."
Several other leading senators also said they were in the dark about the operation in the western Africa nation.
"I didn't know there was 1,000 troops in Niger," Sen. Lindsey Graham, R-South Carolina, told NBC's Chuck Todd on "Meet the Press" Sunday. "They are going to brief us next week as to why they were there and what they were doing."
He continued: "I got a little insight on why they were there and what they were doing. I can say this to the families: They were there to defend America. They were there to help allies. They were there to prevent another platform to attack America and our allies."
Graham also said during the interview that his longtime friend and colleague in the Senate, Arizona Republican Sen. John McCain, is frustrated, "rightly so."
"We don't know exactly where we're at in the world, militarily, and what we're doing. So John McCain is going to try to create a new system to make sure that we can answer the question (about) why we were there," he said. "We'll know how many soldiers are there, and if somebody gets killed there, that we won't find out about it in the paper."
When Senate Minority Leader Chuck Schumer was asked later on "Meet the Press" about knowing whether there were troops in Niger, he responded, "No, I did not."
The Pentagon told CNN it keeps Congress regularly informed on the movements within the agency. And the White House said Monday it notified congressional leaders in June about 965 troops conducting counterterrorism duties
in Niger and Cameroon.
Several Republican House members told CNN on Monday they were aware of the operation.
"With respect to Niger, I serve on the appropriations committee. I oversee military construction projects," Rep. Charlie Dent, R-Pennsylvania, told Cuomo. "We have a presence there. Not just there, but within that whole Lake Chad region, supporting local troops to support fight Boko Haram, support operations in West Africa and the operation in Mali. So we have all sorts of people in that region fighting a very dangerous foe, and ISIS in West Africa, especially."
And Oklahoma Republican Rep. Steve Russell, a member of the government oversight and reform committee, also told CNN he knew about the troops.
"It's not new, and lawmakers that seem to be aghast at these missions going on are simply not well-read," Dent told CNN's John Berman and Poppy Harlow.