The governor’s office, Mr. Mercer said, was “on an island,” and aides prepared three options: a denial, an admission and a condemnation of the photo coupled with a pledge to investigate it. Aides discarded the third choice, Mr. Mercer recalled, because “you can’t equivocate whether you were in the picture or not.”

The group collectively opted for a statement in which Mr. Northam would effectively admit to appearing in the photograph. Aides drafted the statement, which Mr. Northam reviewed and approved.

The statement — in which Mr. Northam said he was “deeply sorry for the decision I made to appear as I did in this photo and for the hurt that decision caused then and now” — contradicted what Mr. Northam was telling his staff, according to the investigators. But a panic, Mr. Mercer said, had seized the governor’s office.

“I wanted to take responsibility for a picture being on my yearbook page and — I hate to say this because I’ve been in the Army and taken care of dying children — but I was shocked,” the governor said, according to the report. “It hit me like a ton of bricks. I didn’t think through it.”

As hours passed, investigators said, Mr. Northam became more confident that he was not in the photograph. It was also becoming clear that Mr. Northam’s initial statements, one in writing and another on video, would not quell the outrage. The governor scheduled a news conference that would shadow him for months: He denied being in the photograph, he admitted to once darkening his face for a Michael Jackson costume and, at one moment, he seemed poised to demonstrate his dance skills.

Mr. Northam and his staff, Mr. Mercer said, “underestimated how strong and how quickly” there would be demands for his resignation.

Within days, though, scandals were enveloping Mr. Fairfax, Mr. Herring and a powerful Republican senator. The governor retreated from public view and re-emerged slowly. He cannot run for re-election in 2021, and in Richmond, people have become increasingly convinced that he will serve — hobbled as he might be — the balance of his term.