The revelation of the photograph prompted some Republicans to call for his resignation. Northam quickly apologized.
The Democratic governor confirmed in a statement that he was in the photo wearing a costume "that is clearly racist and offensive." Northam did not say which costume he was wearing.
My fellow Virginians, earlier today I released a statement apologizing for behavior in my past that falls far short of the standard you set for me when you elected me to be your governor. I believe you deserve to hear directly from me. pic.twitter.com/1rSw1oxfrX— Ralph Northam (@GovernorVA) February 2, 2019
"This behavior is not in keeping with who I am today and the values I have fought for throughout my career in the military, in medicine, and in public service. But I want to be clear, I understand how this decision shakes Virginians' faith in that commitment," he said.
Republican state Sen. Bryce Reeves said in a statement that Northam should resign if the reports of the photos are accurate.
"I hope that this picture is inaccurate and that the Governor brings clarity to this issue. This has no place in Virginia," Reeves said.
Former Obama housing secretary Julian Castro, a Texas Democrat who has announced he will run for president in 2020, tweeted that Northam should resign.
"It doesn't matter if he is a Republican or a Democrat. This behavior was racist and unconscionable," he wrote.
If Northam were to resign, Lt. Gov. Justin Fairfax, who is African-American, would assume the governor's office.
For now, Northam's close allies haven't said anything publicly. Democratic Sen. Jennifer McClellan, a well-known African-American lawmaker from Richmond, just shook her head when approached by a reporter seeking comment.
The Virginian-Pilot obtained a copy of the photo Friday from Eastern Virginia Medical School library, which Northam attended. The photo shows two people looking at the camera - one in blackface wearing a hat, bow tie and plaid pants; the other in white Klan robes.
The yearbook images were first published by the conservative news outlet Big League Politics. An Associated Press reporter saw the yearbook page and confirmed its authenticity at the medical school.
Northam, a folksy pediatric neurologist who is personal friends with many GOP lawmakers, has recently come under fire from Republicans who have accused him of backing infanticide after he said he supported a bill loosening restrictions on late-term abortions.
Last week, Florida's secretary of state resigned after photos from a 2005 Halloween party showed him in blackface while dressed as a Hurricane Katrina victim.
Associated Press writer Ben Finley contributed to this report.
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