In the message, which Mr. Dees showed to a reporter, Mr. Cohen cited Mr. Dees’s “conduct” and the “statements you’ve made in the presence of your co-workers.”

The dismissal, it read, was “for the good of the center.”

On Friday, after The Times asked the S.P.L.C. to respond to its findings, the center offered its fullest account yet of what led to Mr. Dees’s downfall, but it largely refused to address specific allegations or personnel matters.

“We will not respond to individual allegations publicly,” said the center, which declined to make Mr. Cohen available for an interview.

Within hours of the center disclosing the two investigations into Mr. Dees, Mr. Cohen told employees that he would step down. He said in an internal email, “Whatever problems exist at the S.P.L.C. happened on my watch, so I take responsibility for them.”

Mr. Dees said he did not bear a grudge against the organization he helped power for decades, but that he was saddened that his critics “would go to extreme measures” to undermine his career. “I guess I’m a trial lawyer, and so things come and go,” Mr. Dees said. “When the facts are all said and done, my reputation is not going to be hurt.”

The center’s board has hired Tina Tchen, Michelle Obama’s former chief of staff, to assess its workplace culture. In a statement, the board chairman, Bryan Fair, said that recent events had been “an eye-opening reminder that the walk toward justice must sometimes start at your own front door, and force you to look at your past so you can improve your future.”