Bianco's name first emerged in connection with the Oath Keepers through a data hack of the organization's website, first reported by USA Today.
Hackers then made public the names and addresses of purported members of the organization, including what USA Today reported as more than 200 active or retired law enforcement officials.
Some members of the group are currently facing charges for their role in the insurrection at the U.S. Capitol on Jan. 6. In the past, Bianco has condemned the actions of those who entered the Capitol.
Bianco said he was a member for one year while he was a lieutenant in the department.
"I paid for a year membership in 2014. I don't even remember. I found an email after all this started coming out," the sheriff responded when contacted by Eyewitness News for comment.
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He defended the group, saying they support the Constitution.
"I've given a couple statements so far. I guess my point would be that what happened on Jan 6 by a few people, should have nothing to do with something I did in 2014 with an organization that supports the Constitution," Bianco told Eyewitness News.
"I, along with every law enforcement officer and politician, including the president, swear an oath to uphold the Constitution against all enemies," Bianco stated. "Only in today's politically polarized environment, does being a proud American who supports the Constitution make you a right-wing extremist. The hypocrites who cry that one bad apple doesn't mean everyone is bad, now want everyone bad because of a few bad apples."
Bianco is also upset about the hack.
"Where is the FBI on the investigation of the hack? I won't hold my breath for that investigation. Or that media coverage," Bianco said in a statement. "It's probably the same place as their investigation and coverage into Black Lives Matter and Antifa violence and crimes."
Eyewitness News asked Gov. Gavin Newsom about Bianco's former membership in the Oath Keepers.
"That's not a group I want to associate with. That's on our list of hate groups all across this country," Newsom said.
Bianco was elected as Riverside County sheriff in 2018.
He has made headlines for his adamant stances against COVID-19 mandates throughout the pandemic. Last month, he issued a lengthy statement saying he would not enforce a vaccine mandate for sheriff's deputies.
The Council on American-Islamic Relations has called on Bianco to resign.
City News Service contributed to this report.