Last week, in an interview with The New York Times, Rich McHugh, a former producer at NBC News who worked closely with Mr. Farrow, accused the network’s news division of impeding the reporting of the Weinstein story. He called NBC’s handling of the matter “a massive breach of journalistic ethics” and said unnamed higher-ups had ordered the cancellation of an interview with a Weinstein accuser in August of last year, effectively killing the investigation.

Mr. Lack’s email was partly an attempt to defend the network against Mr. McHugh’s statements. In his message to NBC News employees, the chairman publicly defended the network’s handling of the Weinstein story for the first time.

“Contrary to recent allegations, at no point did NBC obstruct Farrow’s reporting or ‘kill’ an interview,” Mr. Lack wrote in the email.

The news executive went on to make the argument that Mr. Farrow’s reporting was not broadcast-ready last August, when he stopped working the Weinstein story for NBC. The main problem, he said, was the lack of on-the-record voices in the version of the story Mr. Farrow had prepared.

“We spent eight months pursuing the story but at the end of that time, NBC News — like many others before us — still did not have a single victim or witness willing to go on the record,” Mr. Lack wrote.