On Feb. 10, Mr. Williams was suspended for six months without pay.

The controversy escalated to the top of the corporation, with Stephen B. Burke, NBCUniversal’s chief executive, playing a crucial role in deciding Mr. Williams’s fate, according to people with knowledge of those deliberations. Also closely involved was Mr. Lack, who was president of NBC News from 1993 to 2001. Mr. Lack is known to be close to Mr. Williams, and his hiring was viewed as a signal that NBC was trying to figure out a way to keep Mr. Williams at the network in some capacity.

Almost immediately after the controversy erupted, NBC opened an investigation into Mr. Williams, led by Richard Esposito, the senior executive producer for investigations. Over the last several months it uncovered 10 to 12 instances in which he was thought to have exaggerated or fabricated accounts of his reporting, according to people familiar with the inquiry. Mr. Esposito declined to comment Wednesday night.

The investigation was conducted in an almost legalistic fashion, said one person familiar with the process, who declined to be identified because of the sensitivity of the inquiry. There was one work room that had printouts on the walls of online pictures spoofing Mr. Williams and his purported tall tales, the person said, but over all the investigation was rigorous and detailed. It generated hundreds of pages of material, based partly on interviews with Mr. Williams’s colleagues inside NBC who had firsthand knowledge of his reporting.

Television industry executives said that it would have been difficult for Mr. Williams to return to the anchor chair because he had lost the trust not only of viewers but also the people in NBC’s newsroom. And, in fact, some of Mr. Williams’s colleagues had brought suspicious instances of reporting by Mr. Williams to the attention of investigators and had provided them with information. It is not clear whether Mr. Williams is aware of the investigation’s contents.

A person close to Mr. Williams who spoke on condition of anonymity because the discussions were private, said his supporters saw the decision as, if not quite an exoneration, a signal that any wrongdoing investigated by NBC was not so egregious as to prevent a return. When the scandal first emerged, many media executives inside and outside the company had thought it was untenable for him to remain with the company.

Industry executives said the decision most likely involved more than journalistic ethics, with hundreds of millions of advertising dollars at stake. NBC News is in a fierce competition with rival news groups for ratings, which affects how advertisers allocate their dollars. NBC generated $200 million in ad sales for its evening news broadcast in 2013, according to Kantar Media.