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Lester Holt, the veteran NBC newscaster and anchor of the “NBC Nightly News” over the last decade, announced on Monday that he will step down from the flagship evening newscast in the coming months.

Mr. Holt told colleagues that he would remain at NBC, expanding his duties at “Dateline,” where he serves as the show’s anchor.

“It has truly been the honor of a lifetime to work with each of you every day, keeping journalism as our true north and our viewers at the center of everything we do,” Mr. Holt, 65, wrote in a memo to colleagues.

He said that he would continue anchoring the evening news until “the start of summer.” The network did not immediately name a successor.

Mr. Holt provided a steady hand to “NBC Nightly News” when he ascended to the job in 2015. The evening newscast was mired in a scandal after it was revealed that Brian Williams, the anchor at the time, had embellished a story about a helicopter attack in Iraq.

Mr. Holt, then the weekend anchor of the “NBC Nightly News,” filled in for Mr. Williams for several months, and then was given the job full time after it was decided that Mr. Williams would move over to MSNBC after a suspension.