Washington (CNN) Virginia Democratic Sen. Tim Kaine said Wednesday that "people will be held accountable" for unauthorized portions of a mission involving Americans soldiers in Niger, four of whom were later killed in an ambush.

Kaine, who sits on the Senate Armed Services Committee and the Foreign Relations Committee, told CNN's "New Day" that during a briefing Tuesday, he and other senators learned that the troops initially attempted to embark on a search for a high value target, a mission they were not authorized, trained or equipped to conduct. They later reverted to an advisory capacity with the Nigerien forces they were working with.

"I believe that the troops who were sadly killed in Niger in October of 2017 were engaged in a mission that they were not authorized by law to participate in and that they were not trained to participate in. And that is a significant reason that they tragically lost their lives," he added.

Kaine refrained from further details about the mission because of classified nature of the briefing.

The Virginia senator told CNN that other senators left the briefing "somewhat shocked."

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