Story highlights Several leading senators also said they were in the dark about the operation

The Pentagon said it keeps lawmakers regularly updated

Washington (CNN) Some senators are saying they didn't know the US had troops in Niger as questions swirl about the raid that killed four US servicemen there earlier this month.

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."

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