News on Wednesday disclosed that it had "received a detailed complaint from a colleague about inappropriate sexual behavior in the workplace" by Lauer.
An NBC News source said Friday that Lauer will not be paid past his last day of work, effectively Tuesday, when Lack informed him he was sacked.
Mr. Lack said in the memo, a "team of the most experienced NBCUniversal Legal and Human Resources leaders have begun a thorough and timely review of what happened" regarding Lauer who is being accused of sexual misconduct.
Lauer also said repairing the damage he had caused was now "his full-time job". "We can say unequivocally, that, prior to Monday night, current NBC News management was never made aware of any complaints about Matt Lauer's conduct", a spokeswoman said.
"At the conclusion of the review we will share what we've learned, no matter how painful, and act on it", he continued.
NBC is faced with the task of suddenly replacing the man who has been the most visible figure in morning television news, the most lucrative part of the network news business.
Reiterating comments from a memo he sent Wednesday, Lack said the top priority is to create a safe workplace environment and that unacceptable actions are "met with consequences, no matter who the offender".
"This week we saw that when an employee comes forward to report misconduct, the system works".
Lauer, 59, issued a statement that was read Thursday on the show he anchored for two decades, in which he said he was "truly sorry". But workers must be empowered to "take the crucial first step of reporting bad behaviour".
Last fall, the network was accused of sitting on an "Access Hollywood" audio tape in which then-candidate Donald Trump was heard telling Billy Bush that prominence came with a license to grope women.
Today show host Matt Lauer has apologised for what he called his "troubling flaws" in a statement read out on the popular NBC morning show on Thursday, a day after the network fired him for inappropriate sexual behaviour.
O'Reilly's ouster from the network came in April, after The New York Times reported that five women were paid a total of $13 million to settle harassment suits.