The faces of 30 white men, all speakers at an applied history conference this month at Stanford University, certainly grabbed attention as the images circulated widely last week.

And that's how the main organizer, conservative British historian Niall Ferguson, a senior fellow at the Hoover Institution, found himself on the defensive two weeks after the conference had ended.

Propelling the blowup was a Twitter post about the "all-male history conference" posted Wednesday by Ana Lucia Araujo, a history professor at Howard University in Washington.

"This goes for the Guinness Book of the century!" she wrote above the roster showing "a team of 30 white male historians" listed as panelists and speakers at the conference on the campus near Palo Alto, California. "What a shame."

The conference in the heart of Silicon Valley had actually been held March 2 and 3, with a dozen sessions, including ones dedicated to questions like "Are recent developments in American politics unprecedented, or is Trump merely populism revisited?" and "What can we learn from past attempts to learn from the past?"

Ferguson is no stranger to controversy. But on Thursday he acknowledged the conference needed more diversity.

"Everybody was keenly aware that it was too white and too male," he said. The problem was "explicitly discussed" before the event, Ferguson said. He also said women had been invited, but all, except one, were unable to attend.

Three female historians who had been invited confirmed to The New York Times that they had declined because of other obligations. The only woman at the event, Mary Sarotte, who headed a single panel, did not respond to a request for comment.

After the interview, Ferguson reiterated his defense in a series of Twitter posts.

"One speaker made a forceful critique in his prepared remarks," he said. "We all agreed that we must redouble our efforts to represent diverse viewpoints in future conferences."

Not everyone felt the outcome stemmed solely from the growing pains of a startup conference.

"Given how prevalent women are in the history department, you'd have to try really hard to come up with a roster of speakers that looks like that," Priya Satia, a Stanford history professor and the head of the department's diversity committee, said Friday.

Some fields are notoriously male-dominated, like science and technology, she said, but history is different. "There are a healthy amount of women," she said.

The Stanford history department did not have a role in arranging the conference, but Satia said she was still astonished to learn about its existence only two days before it was "happening in the building next door."

She became aware of it, she said, when a colleague posted the image of the all-male roster. The image was not public on the event's website.

The secrecy and lack of advertising were peculiar and didn't seem in the interest of "sharing and producing knowledge," Satia added.

As for Ferguson's comment that the organizers were concerned about the demographics, she said, "Should he have gone forward with a conference that looked like that?"

Lisa Lapin, vice president for communications at Stanford, said the university was aware of the efforts to attract a more diverse group to the conference and had encouraged the organizers to take additional steps in the future. About 20 of the 87 people at the event were women, she said.

Araujo, whose Twitter post of the roster was widely shared this week, declined to comment but pointed to a smaller Stanford event in January on the intersections of politics and technology. Only white men were on stage. Two were Silicon Valley billionaires: Peter Thiel, chairman of Palantir and a prominent Trump supporter, and Reid Hoffman, the LinkedIn founder who says he wants to reinvent the Democratic Party. Ferguson moderated.

Satia also expressed concerns about the Hoover Institution's track record on diversity, noting an acceptance speech Ferguson gave in 2016 for the Philip Merrill Award for Outstanding Contributions to Liberal Arts Education, in which he criticized the growth in historians specializing in gender and race.

For his part, Ferguson was still apologetic Friday, saying in an email reply to a follow-up question: "I have no doubt that there are many talented female historians whom I should have invited. I reproach myself for not knowing them and not having done more to get to know them. If any good has come of all this negative publicity, perhaps I shall now get to know them."

He added: "Or perhaps the result will instead be a boycott of future events I organize. We shall see."