Brian Williams' claims probed by NBC's investigative unit

NBC News has assigned the head of its own investigative unit to look into statements made by anchor Brian Williams about his reporting in Iraq a dozen years ago.

A source at the network who requested anonymity because the person is not authorized to speak on personnel matters confirmed the investigation on Friday. Williams has apologized for falsely saying on the air that he was in a helicopter hit by a rocket-propelled grenade while in Iraq in 2003.

Richard Esposito, a former editor at the New York Daily News now at NBC, will head the investigation. The incident has ballooned into a full-blown crisis for NBC and Williams, whose "Nightly News" is the top-rated evening news program.

In 2003, Williams described how he was traveling in a group of helicopters forced down in the Iraq desert. On the ground, Williams said, he learned the Chinook in front of him "had almost been blown out of the sky;" he showed a photo of the aircraft with a gash from a rocket-propelled grenade.

In a 2008 blog post, Williams said that his helicopter had come under fire from what appeared to be Iraqi farmers with rocket-propelled grenade. He said a helicopter in front of his had been hit.

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Then, in a 2013 appearance on David Letterman's "Late Show" on CBS, Williams said that two of the four helicopters he was traveling with had been hit by ground fire, "including the one I was in."

On Wednesday, Williams recanted that story, claiming that he was flying in a Chinook helicopter behind the formation that took fire. However, on Thursday, the military newspaper Stars and Stripes, which broke the story, reported that Williams was actually flying with a different helicopter company altogether, in a different direction, and linked to the attacked unit only by radio.

The Associated Press contributed to this report.