Virginia Governor Ralph Northam has denied he was in a racist photo that appeared in his 1984 student yearbook page after initially apologising for it.
Mr Northam said he recognised that people would find this "hard to believe".
He also admitted blackening his face to impersonate singer Michael Jackson at an event in the same year.
Top US Democrats have called on him to resign. But he says he will stay on.
The picture showed a man in blackface and another man in Ku Klux Klan robes.
Former Vice-President Joe Biden said Mr Northam had lost all moral authority. Black politicians in Virginia called it "disgusting" and Republicans also urged him to resign.
What did Northam say?
He said he had initially taken responsibility for the photo, which he described as "clearly racist and offensive".
But he said that on reflection with his family and friends he had concluded that he was not either of the people in the photo.
"It has taken time to make sure that it's not me but I'm convinced I'm not on that photo," he said.
He said that he had however blackened his face at a dance contest in San Antonio - "It is because my memory of that is so vivid that I do not believe I am in the photo in the yearbook," he said.
Mr Northam said he was not asking for forgiveness for his past actions but for "the opportunity to demonstrate without a shadow of a doubt that the person I was then is not the person I am today".
The statement on Saturday came after he said on Friday that 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".
How has this come about?
Mr Northam's yearbook page, which came from the paediatric neurologist's time at Eastern Virginia Medical School, was first published by conservative website Big League Politics.
The Virginian-Pilot newspaper tweeted a picture of the page which it said it obtained from the medical school library.
BREAKING: Gov. Ralph Northam yearbook page shows blackface and Klan photohttps://t.co/6A89ejp5Ho— The Virginian-Pilot (@virginianpilot) February 1, 2019
An official from the medical school verified the photo and told the Huffington Post that it had come from a "student-produced publication".
What reaction has there been?
The Virginia Legislative Black Caucus, which comprises African Americans elected to the state legislature, described the images as "disgusting, reprehensible and offensive".
"These pictures rip off the scabs of an excruciatingly painful history and are a piercing reminder of this nation's sins. Those who would excuse the pictures are just as culpable," it said in a statement.
In a tweet, Mr Biden said Mr Northam should resign immediately.
There is no place for racism in America. Governor Northam has lost all moral authority and should resign immediately, Justin Fairfax is the leader Virginia needs now.— Joe Biden (@JoeBiden) February 2, 2019
Calls for his resignation also came from four Democratic candidates for president - Senators Kirsten Gillibrand, Kamala Harris and Cory Booker, and Texan mayor Julian Castro.
The photo also prompted a swift backlash from conservatives, including Jack Wilson, the chairman of the Republican Party of Virginia, who called on Mr Northam to step down.
"Racism has no place in Virginia," he said in a statement. "These pictures are wholly inappropriate. If Governor Northam appeared in blackface or dressed in a KKK robe, he should resign immediately."
These images arouse centuries of anger, anguish, and racist violence and they’ve eroded all confidence in Gov. Northam’s ability to lead. We should expect more from our elected officials. He should resign.— Cory Booker (@CoryBooker) February 2, 2019
Democratic Senator Elizabeth Warren, who is considering a presidential run in 2020, also called for Mr Northam to resign.
These racist images are deeply disturbing. Hatred and discrimination have no place in our country and must not be tolerated, especially from our leaders – Republican or Democrat. Northam must resign.— Elizabeth Warren (@ewarren) February 2, 2019
The Ku Klux Klan is one of the oldest and most infamous hate groups in the US, and has targeted African Americans, Jews, Catholics and immigrants, according to the Southern Poverty Law Center, which tracks hate groups.
The group has a long history, with KKK membership peaking in the 1920s.