The investigation found that leadership at the network did not know about Matt Lauer's behavior prior to the anonymous woman who came forward in November, prompting his firing

Over five months after he was fired by NBC for alleged sexual misconduct, the network has released the findings of its internal investigation into former Today show anchor Matt Lauer.

The investigation, which was led by NBCUniversal general counsel Kim Harris, focused on four complainants’ allegations and resulted from nearly 70 interviews with current and former employees and over 30 focus groups with 262 current employees.

The investigation team concluded that they “found credible the four complainants’ allegations that Lauer engaged in inappropriate sexual behavior in the workplace.”

The investigation also found that leadership at NBC News, Today and human resources were not made aware of Lauer’s behavior prior to the anonymous woman who came forward in November, prompting his firing.

“We found no evidence indicating that any NBC News or Today show leadership, News HR or others in positions of authority in the News Division received any complaints about Lauer’s workplace behavior prior to November 27, 2017,” the report states. “All four women who came forward confirmed that they did not tell their direct manager or anyone else in a position of authority about their sexual encounters with Lauer. Current and former members of NBC News and Today show leadership, as well as News HR, stated that they had never received a complaint about inappropriate workplace behavior by Lauer, and we did not find any contrary evidence.”

However, “two of the four complainants who came forward said that they believe former NBC News or Today Show leadership knew or must have known about Lauer’s alleged inappropriate sexual behavior in the workplace. The former leaders with whom we spoke denied any such knowledge.”

The investigation team determined that while some witnesses had concerns about reporting inappropriate behavior to human resources, they don’t believe that there is a “widespread or systemic pattern of behavior that violates company policy or a culture of harassment in the News Division.”

An insider previously told PEOPLE that Lauer was let go due to sexual misconduct throughout 2014 with a female staffer, including at the Winter Olympics in Sochi, Russia. Women have also anonymously accused Lauer of sexual harassment and assault in reports published by Variety and The New York Times.

The investigation also addressed a much-discussed topic in the wake of Lauer’s firing: a so-called “secret button” under his desk that Lauer allegedly used to lock his door inside NBC’s Rockefeller Center headquarters.

“The button is a commonly available feature in executive offices in multiple NBCU facilities to provide an efficient way to close the door without getting up from the desk,” according to the report. “The button releases a magnet that holds the door open. It does not lock the door from the inside.” Finally, the investigation touched on the Washington Post article in which Lauer’s former co-anchor Ann Curry said she approached two members of NBC’s management team in 2012 after a female staffer at the network told her she was “sexually harassed physically” by him.

“Curry confirmed that she did not disclose to anyone in management that she had received a specific complaint,” according to the report. “Curry declined to share with the investigation team the identity of anyone in management with whom she spoke at the time or the identity of the woman who came to her with a complaint about Lauer. The members of NBC News and Today show leadership at the time with whom we spoke denied having any such conversation with Curry.”

In a separate memo to employees sent on Wednesday morning obtained by PEOPLE, news division chairman Andy Lack admitted that “the last few months have been extraordinarily difficult.”

“When we first told you about the decision to dismiss Matt Lauer, I said we had been presented with ‘reason to believe this may not have been an isolated incident,’ ” he said. “As you will see in the report, that turned out to be the case.”

“Like many of you, I am immensely proud of NBC News, its history, and the work we do. But — stepping back from the investigation — that history also includes a time when people were not comfortable coming forward to voice complaints about repugnant behavior. That is not acceptable,” he continued. “We cannot change the past. What we can do is learn from it, and try to make it right.”

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Last month, Lauer released a statement in regards to the allegations published by the Washington Post.

“Five months ago I was terminated by NBC after admitting to past relationships with co-workers,” he said. “A day later I took responsibility, apologized to the people I had hurt and promised to begin the process of repairing the damage I had caused my family. I have worked every day since then to honor that promise.”

“I have made no public comments on the many false stories from anonymous or biased sources that have been reported about me over these past several months — including a claim that I would, or even could, lock someone in my office. I remained silent in an attempt to protect my family from further embarrassment and to restore a small degree of the privacy they have lost,” he continued. “But defending my family now requires me to speak up. I fully acknowledge that I acted inappropriately as a husband, father and principal at NBC. However I want to make it perfectly clear that any allegations or reports of coercive, aggressive or abusive actions on my part, at any time, are absolutely false.”

RELATED VIDEO: Matt Lauer & Wife Annette Are ‘Preparing’ to Divorce as Source Says the ‘Damage Can’t Be Fixed’

Since being fired, Lauer has been lying low in the Hamptons — and a source close to him recently told PEOPLE that he remains “truly devastated” over the situation that ended his career at the network.

A source told PEOPLE this week that Lauer and his wife of 20 years Annette Roque are “barely speaking.” They share three kids together.