NBC News host Brian Williams has "backed out" of a scheduled appearance on David Letterman's Late Show.

The news came from NBC on Sunday, after Williams announced he was stepping away from NBC's "Nightly News" while the network "looks into the anchor's admission that he had told a false story about being on helicopter hit by a grenade while reporting on the Iraq war."

Williams had spread the story of his helicopter being under attack during a previous "Late Show" interview with Letterman. He apologized for the statements on Wednesday, and NBC News launched an internal investigation Friday.

NBC News has refused to comment on when or whether Williams will return and who will decide his future.

Since Williams apologized about the helicopter story, other questions are surfacing about claims he made about Hurricane Katrina, where he claimed he saw bodies floating in the floodwaters while in New Orleans.

His remarks in a 2006 interview drew suspicion because there was relatively little flooding in New Orleans' French Quarter, the area where Williams was staying. A person at NBC confirmed that Williams stayed at the Ritz-Carlton, which is in an area where a news photographer and a law enforcement official said they saw bodies.

Lester Holt will be filling in while Williams is out.