NEW YORK -- NBC News anchor Brian Williams found himself the story Thursday, his credibility seriously threatened because he falsely claimed he had been in a helicopter hit by a grenade during the Iraq war.

NBC News officials would not say whether their top on-air personality would face disciplinary action. Williams, the "Nightly News" anchor for just over a decade, had become an online punching bag overnight.

Tweets with the hashtag #BrianWilliamsMemories joked that he blew up the Death Star, saved someone from a polar bear and flew with Wonder Woman in her invisible helicopter. Photoshopped pictures showed him reporting from the moon and riding shotgun with O.J. Simpson in his Ford Bronco.

#BrianWilliamsMemories When I turned off my targeting computer and relied on the Force to destroy the Death Star #WhatILearnedToday — Donna (@DMR_MI) February 5, 2015

. @emilylinnert @TODAYshow don't you remember when Brian Williams saved you from a polar bear? #BrianWillliamsMemories — Jay Simon (@jay23simon) February 5, 2015

I REMEMBER THE TIME BRIAN WILLIAMS FLEW WITH WONDER WOMAN TO IRAQ IN HER INVISIBLE HELICOPTER #BrianWillliamsMemories — Melanie Deal (@Elvisfan1976) February 5, 2015

"That one live shot I did from the moon" #BrianWilliamsMemories pic.twitter.com/FbND6jAqc7 — Greg Dutra (@DutraWeather) February 5, 2015

After he said more about remembrance, he passed the bread as I recall. #BrianWilliamsMemories pic.twitter.com/PDlQOZKg6J — Ed Fuentes (@viewfromaloft) February 5, 2015

Correction: I did not run a meth empire while battling terminal cancer, I stayed home w/the flu & ate rock candy. #BrianWilliamsMemories — Marty Lang (@marty_lang) February 4, 2015

Correction: I didn't storm the beach at Normandy. I just had a friend named Norman. #BrianWilliamsMemories — jeremy scahill (@jeremyscahill) February 4, 2015

"How could you expect anyone who served in the military to ever see this guy onscreen again and not feel contempt?" wrote critic David Zurawik, of the Baltimore Sun. "How could you expect anyone to believe he or the broadcast he leads has any credibility?"

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Williams apologized Wednesday for telling the story a week earlier during a "Nightly News" tribute to a veteran he had befriended during a 2003 reporting trip to Iraq. Before expressing his regrets on the air, Williams did so online and in an interview with the newspaper Stars & Stripes.

He speculated online that constant viewing of video showing him inspecting the damaged helicopter "and the fog of memory over 12 years, made me conflate the two, and I apologize."

His story had morphed through the years.

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

The NBC crew and military officials accompanying them spent three days in the desert, kept aground by a sandstorm.

But in a 2008 blog post, Williams said his helicopter had come under fire from what appeared to be Iraqi farmers with RPGs. He said a helicopter in front of his had been hit.

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

"No kidding?" Letterman interjected.

Williams described making a quick, hard landing in the middle of the desert.

"I have to treat you now with renewed respect," Letterman said. "That's a tremendous story."

Williams' story was first questioned in posts to the "Nightly News" Facebook page. It's a touchy topic: Members of the military who are wounded or who come under enemy fire consider themselves members of a special kind of brotherhood and don't like people who try to intrude, said retired U.S. Army Col. Pete Mansoor, a professor of military history at Ohio State University.

"It smacks of stolen valor," Mansoor said - an offense that Williams specifically denied in his online apology.

Chris Simeone, who says he piloted the aircraft that Williams flew in, told The Omaha World-Herald Thursday that "I think it's good that the truth is coming out. But I feel embarrassed for him."

Simeone said if Williams had "just reported the facts, it would have been a great story. He was there. He didn't have to embellish it."

Many people have embellished war stories, in the local tavern or on a national stage. During the 2008 campaign, Hillary Clinton was derided for saying she came under sniper fire when, as first lady, she arrived at a military base in Bosnia. Her representatives said she had misspoken.

Williams' immediate issue is whether people believe his apology, a particular problem in an industry in which credibility is crucial.

"It's pretty difficult to believe," Mansoor said. "I remember every time I was under fire in Iraq, especially if your vehicle is an aircraft that's been hit. That's something that gets seared into your memory for all time."

The New York Daily News labeled Williams' apology a fake.

"So what if it was 12 years ago," wrote TV editor Don Kaplan. "I remember getting hit in the head with a rock by a kid in the third grade."

Williams has an out-sized image at NBC News: The blue-collar Jersey guy and witty celebrity who slow jams the news with The Roots on Jimmy Fallon's "Tonight" show. He hasn't had credibility problems before, and he consistently leads in the ratings, making him an outlier at a network on which the "Today" show and "Meet the Press" have slipped from their lofty perches as ratings champions.

It's up to NBC News President Deborah Turness to decide whether Williams will be punished. She has the reputations of her most well-known personality and the news division as a whole to consider.

Jason Maloni, a crisis public relations expert at Levick Strategic Communications and a regular "Nightly News" viewer, said he isn't sure whether NBC should sanction Williams, but he doesn't want to see the anchor in a war zone anytime soon.

"I feel a little let down, and I imagine that's where a lot of the (online) anger is coming from," Maloni said.

Zurawik, of the Baltimore Sun, wrote that if credibility means anything to NBC News then Williams should be out of his job by the end of the week.

But Jane Hall, a communications professor at American University, said she believes Williams' apology should be accepted.

"It seems to me to be an honest mistake of conflating the two accounts," Hall said. "I don't think he has been known as anything other than a straight shooter."