“NBC Nightly News” anchor Brian Williams' career is in trouble. The 55-year-old's announcement Saturday of a self-imposed hiatus from his top-rated news broadcast was a definitive admission that the public’s rapidly worsening notion of his credibility could no longer be ignored.

Despite his popularity, experts said Williams’ attempts to explain his erroneous claim that he survived a rocket-propelled grenade (RPG) attack on his helicopter while covering the Iraq War in 2003 failed to adequately answer questions about his credibility. His recanted tale led to further inquiries into the truth behind his anecdotes regarding Hurricane Katrina and other major news stories, while Williams’ willingness to promote himself as a pop culture entity has given his many critics plenty of ammunition to attack his journalistic integrity.

“That an anchor should permit himself to become an all-around celebrity simply points to the farcical nature of not just the grandiosity of the anchor but of the absurdity of expecting someone who’s a professional celebrity to also be a truth-teller … I defy anyone to tell me a story that Brian Williams has broken or deepened. He’s not a journalist, he’s masquerading,” Todd Gitlin, a professor and chair of the Ph.D. program at Columbia University’s Graduate School of Journalism in New York, said.

Williams’ troubles began last week, when Stars and Stripes noted that his oft-repeated anecdote that his helicopter was forced down by RPG fire was inaccurate and that the NBC anchor’s helicopter landed well after military aircraft were targeted by rocket fire. As the outcry over the report intensified on the Internet and in media circles, Williams was forced to issue an on-air apology on Wednesday.

“I want to apologize. I said I was traveling in an aircraft that was hit by RPG fire. I was instead in a following aircraft. We all landed after the ground fire incident and spent two harrowing nights in a sandstorm in the Iraq desert. This was a bungled attempt by me to thank one special veteran, and by extension, our brave military men and women, veterans everywhere, those who have served while I did not,” he said.

Williams was right to apologize for his misstep, but the tone of his mea culpa missed the mark, multiple experts said. His failure to provide a full explanation for how he bungled the Iraq story so badly, stating that he didn’t know “what screwed up in my mind that caused me to conflate one aircraft with another,” was a critical error.

“He had a very qualified apology, this whole ‘I don’t know how I conflated this information.’ The only way he could’ve possibly come back from this is if, as soon as somebody called him on this, he had humbly and transparently revealed the truth and apologized profusely and said that he’s learned from this, never intends to do it again, et cetera. That isn’t what he did. He still isn’t doing it,” Jonathan Bernstein, president of Bernstein Crisis Management, said.

Rather than ending questions regarding Williams’ credibility, the Iraq helicopter controversy renewed scrutiny of the longtime anchor’s entire career. By last Friday, several blogs had uncovered video of Williams’ 2006 interview with former Disney chief executive Michael Eisner, during which Williams claimed he saw a body float through New Orleans’ French Quarter after Hurricane Katrina and that he developed dysentery after he accidentally ingested floodwater.

But several eyewitness accounts held the French Quarter never experienced the heavy flooding that crippled the rest of New Orleans, raising doubts about how a dead body could possibly have floated through the neighborhood’s streets. Moreover, a former New Orleans city health official said he could not “recall a single, solitary case of gastroenteritis during Katrina,” according to the New Orleans Advocate. Michael Brown, who led the Federal Emergency Management Agency’s much-criticized response to the hurricane, also took umbrage with Williams’ claims, stating that the French Quarter experienced mere “inches of water” during the storm.

As public criticism and scrutiny mounted, Williams made the decision to step away from his anchor desk while NBC investigated the situation. “In the midst of a career spent covering and consuming news, it has become painfully apparent to me that I am presently too much a part of the news, due to my actions,” Williams said. “As Managing Editor of NBC Nightly News, I have decided to take myself off of my daily broadcast for the next several days, and Lester Holt has kindly agreed to sit in for me to allow us to adequately deal with this issue. Upon my return, I will continue my career-long effort to be worthy of the trust of those who place their trust in us."

Williams also decided to cancel his scheduled appearance this week on “Late Night with David Letterman,” a show on which he’s appeared 21 times in the past. Sources familiar with NBC and Williams’ thinking said the interview -- once considered a key part of Williams’ attempt to regain the audience’s favor -- should not occur during his hiatus from the public eye, according to Politico.

But in a field where credibility is the only real currency, Williams’ continued failure to provide an adequate explanation for critics made his “return” far from certain. Legitimate concerns about Williams’ integrity affect NBC News’ brand as a whole.  

“No mainstream news organization can afford to have people saying ‘your nightly news anchor is a liar.’ You just can’t afford to have it, and it’s not a question of just a couple of crackpots coming out of the woodwork and making an accusation. At least as of now, there appears to be evidence that Brian Williams was not telling the truth,” said Jane Kirtley, a professor of media ethics and law at the University of Minnesota.

Losing Williams would be a blow to NBC. “Nightly News” is a ratings powerhouse, and the network lacks another anchor with the star power to step in and allow for a seamless transition. But if skepticism regarding Williams’ credibility continues, the preservation of the network’s credibility as a whole will take precedence over ratings concerns.

Moreover, having already lost the public trust, it will be difficult for Williams to perform the core function of his job -- namely, to report the news -- regardless of his popularity or celebrity. That’s true even if the public comes to accept that the Iraq story was a one-time mental lapse.

“What kind of credibility is he going to have the next time he deploys himself in a war zone and has to interface directly with the military? Those people are not going to trust him and they’re not going to want to give him their stories and they’re not going to necessarily want him in their helicopters,” said Mark Bernheimer, former CNN correspondent and founder of MediaWorks Resource Group.

With his ability to cover the news permanently impaired, resignation is the only path left to Williams, experts agreed. It’s also the decision he should have made from the start, according to Linda Steiner, a professor at the University of Maryland’s Philip Merrill College of Journalism.

“I don’t think he should wait for critical mass [to resign]. He knows what the right thing to do is. I think he should’ve done that right from the beginning,” she said.