Brian Williams, the nation’s top-rated news anchor, has admitted fabricating a tale of being forced down by enemy fire in a helicopter over Iraq a dozen years ago.

Williams apologized on the air Wednesday evening, saying: “I made a mistake in recalling the events of 12 years ago.”

Earlier, the NBC anchor told Stars & Stripes, which broke the story: “I would not have chosen to make this mistake. I don’t know what screwed up in my mind that caused me to conflate one aircraft with another.”

Williams did not make the claim in reporting on the incident in 2003. But he told the false story on the air in 2013, and lied about it again last Friday.

The admission raises serious questions about his credibility in a business that values that quality above all else. Williams is the longest-serving network anchor, his “NBC Nightly News” has been No. 1 in the ratings for nearly all of the last decade, and his comedic skills have led him to guest-host “Saturday Night Live” and become a regular on the “Daily Show.”

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For such a high-profile journalist to acknowledge that he essentially invented a story that dramatized his bravery in a war zone is hard to fathom. Williams said he had misremembered the story and was sorry.

In a 2013 interview with CBS’s David Letterman, Williams said: "We were in some helicopters. What we didn’t know was, we were north of the invasion. We were the northernmost Americans in Iraq. We were going to drop some bridge portions across the Euphrates so the Third Infantry could cross on them. Two of the four helicopters were hit, by ground fire, including the one I was in, RPG and AK-47." That interview was first noted by the Washington Post.

Williams repeated the tale last week as he was paying tribute at a New York Rangers hockey game to retired soldier who provided security for grounded helicopters while Williams was in Iraq in 2003.

“The story actually started with a terrible moment a dozen years back during the invasion of Iraq when the helicopter we were traveling in was forced down after being hit by an RPG,” the anchor told viewers. “Our traveling NBC News team was rescued, surrounded and kept alive by an armor mechanized platoon from the U.S. Army 3rd Infantry.”

But his account was contradicted by crew members of the 159th Aviation Regiment on board a Chinook copter that was hit by two rockets and small arms fire. They told Stars & Stripes that Williams was nowhere near that helicopter and two other Chinooks in the formation that took fire.

The anchor and his NBC crew arrived in the area west of Baghdad on another helicopter about an hour later, they told the newspaper. The chopper landed because of an Iraqi sandstorm and was grounded by weather for two days.

“No, we never came under direct enemy fire to the aircraft,” Sgt. 1st Class Joseph Miller, the flight engineer on Williams’ helicopter, was quoted as saying.

Some of the crew members on the Chinook that was hit were clearly upset with the anchor’s now-discredited claim. Lance Reynolds, the flight engineer, told Stars & Stripes it had been a “life-changing” trauma and “felt like a personal experience that someone else wanted to participate in and didn’t deserve to participate in.”

NBC trumpeted the Williams saga, airing a story on March 26, 2003, with the headline: “Target Iraq: Helicopter NBC’s Brian Williams Was Riding In Comes Under Fire.” But he did not claim to be in the downed chopper, saying: “On the ground, we learn the Chinook ahead of us was almost blown out of the sky.”

In a 2008 blog post, he again wrote that the “Chinook helicopter flying in front of ours (from the 101st Airborne) took an RPG to the rear rotor.”

That makes it even more difficult to understand why Williams would repeat the claim last week, only to have it shot down by those who were there.

In a Facebook comment, Williams said: “I spent much of the weekend thinking I'd gone crazy.  I feel terrible about making this mistake, especially since I found my OWN WRITING about the incident from back in '08, and I was indeed on the Chinook behind the bird that took the RPG in the tail housing just above the ramp.  Because I have no desire to fictionalize my experience (we all saw it happened the first time) and no need to dramatize events as they actually happened, I think the constant viewing of the video showing us inspecting the impact area -- and the fog of memory over 12 years -- made me conflate the two, and I apologize.”

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Howard Kurtz is a Fox News analyst and the host of "MediaBuzz" (Sundays 11 a.m. and 5 p.m. ET). He is the author of five books and is based in Washington. Follow him at @HowardKurtz. Click here for more information on Howard Kurtz.