The investigation also found the nonprofit agency, one of the country's biggest animal charities, had offered settlements to three other workers who said they were demoted or dismissed after reporting Pacelle's alleged behavior, according to the Humane Society memo.
Investigators from the law firm Morgan Lewis, who interviewed 33 people, including Pacelle, also reported that there was a perception within the Humane Society that certain women owed their career success to romantic relationships with the chief executive.
The memo said that several former high-ranking women had warned Pacelle, who has led the organization since 2004, that his sexual relationships with subordinates, donors and volunteers could hurt the charity. The memo notes that Pacelle, while not directly addressing the issue, said he had changed his behavior as he grew older.
Pacelle denied the complaints from all three women in an interview Monday with The Washington Post. "This is a coordinated attempt to attack me and the organization," he said. "And I absolutely deny any suggestion that I did anything untoward."
He denied allegations he had consensual sex with donors and volunteers as "just ad hominem attacks." And he said no senior women had warned him about his conduct. "Absolutely not. I enjoy the support of senior women throughout the organization. No one has ever warned me of such a thing, ever."
A spokesperson for the Humane Society declined to comment on the findings of the investigation, referring The Post to a statement made Thursday by Eric Bernthal, chairman of the organization's board of directors. His remarks came shortly after the charity announced it was launching an investigation.
"We do not have information that can be shared regarding the investigation, its findings, or board actions at this time," the statement said. "We believe it is important to deal in substance and not rumors, and our process is designed to ensure confidentiality and fair consideration of these issues."
Pacelle said that he was aware of the investigation but added: "There are allegations only. Beware of rumors and other unsubstantiated claims."
The earliest complaint against Pacelle dates to 2005, when an intern said the chief executive asked to meet outside of work at a public coffee shop, according to the memo reviewed by The Post.
According to her account, Pacelle pulled her close, started slow dancing with her and gave her an unwanted kiss, the investigation document said.
Another woman told investigators she regularly traveled with Pacelle on business and that donors mentioned his sexual interest in her, remarking that dating him would be good for her career.
On one such work trip in 2006, she told investigators Pacelle asked her to stop by his hotel room after an event. He asked if he could masturbate in front of her, requested that she take off her clothes and offered to perform oral sex on her, according to two people briefed on the matter and the memo. When the woman refused, Pacelle told her not to tell anyone or she would destroy the Humane Society and lose her job, according to the memo.
A third woman, who joined the Humane Society in 2012 but has since left the organization, told investigators that Pacelle stopped by her office late one night when she was working alone, started salsa dancing on his own and asked her to join him.
Pacelle denied all three complaints. "The one complaint about the salsa dancing, I simply had a conversation with a person and it turned into that," he told The Post. "The person with the hotel — I'm familiar with that. I worked with the person eight years after that allegation. The person never said a thing to me about any harassment, and I certainly never invited her to a hotel room."
Pacelle also denied the complaints to investigators, according to the memo.
The investigation began on Dec. 20 after the Humane Society received an anonymous complaint about Pacelle's behavior. The organization then hired Morgan Lewis, a Washington law firm, to look into the matter.
The investigation also found that Pacelle had maintained a sexual relationship with a female subordinate and exchanged more than 100 emails with her. The woman told the investigation that she became afraid of Pacelle after the relationship ended, describing him as abusive and controlling, according to the memo.
Pacelle denied having a relationship with the subordinate. He also disputed there was anything inappropriate in the relationship to investigators, according to the memo.
The investigation's findings are based on interviews, evidence provided by witnesses and emails on Pacelle's work computer, according to the memo.
Since taking the helm at the Humane Society, which has its headquarters in the District, Pacelle's salary rose to nearly $380,000 in 2016, according to IRS filings.
Some employees defended Pacelle to investigators, describing him as someone who engaged in consensual relationships with adults. Others said the chief executive created a toxic environment at the Humane Society in which workers thought they had to sleep with Pacelle to get ahead, or suspected women who achieved career success of dating him in secret, according to the memo.
The people briefed on the investigation told The Post they wanted to come forward to repair the culture at the Humane Society, which they believe does important work to help animals.
The organization is focused on ending animal cruelty, abolishing "puppy mills" and banning seal slaughter, among other causes.
The people briefed on the investigation said they worried that money going to address Pacelle's actions was misdirected from protecting wildlife.
One woman said she received a settlement from the Humane Society after she complained about Pacelle's alleged girlfriend joining her team without proper qualifications and was shut out of work opportunities, according to the memo.
Two more received payouts after they leveled retaliation charges against the organization, asserting they lost their jobs after speaking up about Pacelle's office romance and sexual behavior in the office, according to the memo.
Pacelle said he would not comment on any settlements.
The amounts of the settlements were not disclosed.