Earlier this week, Brian Williams admitted to lying about flying in an aircraft hit by a rocket-propelled grenade while reporting from Iraq in 2003. Now, his journalistic credibility is under scrutiny and his reporting is under an internal investigation.

Richard Esposito, the head of NBC's investigative unit, will lead the investigation against the "NBC Nightly News" anchor, according to New York Daily News. The probe will include his Iraq story as well as his reporting during Hurricane Katrina in 2005.

Williams' account of seeing a body float by his hotel in the French Quarter of New Orleans days after Hurricane Katrina has come into question given the lack of flood waters in that area. However, new photos obtained by The New Orleans Advocate show there was indeed water outside the Ritz-Carlton where the NBC anchor was staying.

He also claimed he fell ill with dysentery by ingesting flood water. A spokeswoman from the Louisiana Department of Health and Hospitals told the Advocate that it does not track cases of dysentery, but contaminated water sources are possible "transmission points" for inflammation of the intestines.

Both claims are "likely unverifiable," according to the Advocate.

Former NBC anchor Tom Brokaw also denied reports that he wanted Williams fired.

"I have neither demanded nor suggested Brian be fired," Brokaw told The Huffington Post. "His future is up to Brian and NBC News executives."

With the Iraq story, Williams falsely claimed that his chopper had been hit by an RPG. In reality,  the helicopter in front of his that was hit. He told the story most recently during NBC's coverage of a public tribute at a New York Rangers hockey game for a retired soldier who provided ground security for the helicopters when they were forced down in the Iraq desert. He recanted the story during the Feb. 3 broadcast of "Nightly News."

"I made a mistake in recalling the events of 12 years ago. It did not take long to hear from some brave men and women in the air crews who were also in that desert," Williams stated. "I want to apologize... This was a bungled attempt by me to thank one special veteran and by extension our brave military men and women."

Along with his apology on air, Williams also apologized to his NBC News colleagues. NBC News president Deborah Turness sent out a staff-wide memo to address the current upheaval in the newsroom.

Read the memo below:

All,

This has been a difficult few days for all of us at NBC News.

Yesterday, Brian and I spoke to the Nightly News team. And this morning at the Editorial Exchange, we both addressed the wider group. Brian apologized once again and specifically expressed how sorry he is for the impact this has had on all of you and on this proud organization.

As you would expect, we have a team dedicated to gathering the facts to help us make sense of all that has transpired. We're working on what the best next steps are - and when we have something to communicate we will of course share it with you.

Since joining NBC News, I've seen great strength and resilience. We are a close-knit family, and your response this week has made that even clearer.

As a relentless news agenda marches on, thank you again for continuing to do what we do best - bring the most important stories of the day to our audience.

Deborah