Jason Van Tatenhove, a former Oath Keepers spokesman, now talks about its dangers.

One witness appearing at the House select committee’s hearing on Tuesday will likely give an insider’s glimpse of the Oath Keepers militia: Jason Van Tatenhove, a former spokesman for the group who left it several years ago and has more recently been speaking out about the dangers it presents.

Mr. Van Tatenhove is expected to testify about the history of the Oath Keepers, which was founded in 2009 by a former Army paratrooper, Stewart Rhodes, and how the group became increasingly radical in the months and years leading up to the Capitol attack, according to a person familiar with the matter. Mr. Van Tatenhove’s testimony will also likely touch on how Mr. Rhodes used conspiracy theories to raise money for the group and increase its membership.

Mr. Van Tatenhove began to work with the Oath Keepers around 2014, eventually becoming the public face of the group during several prominent anti-government and militia operations. He issued statements for the organization — and answered questions from the media — after Mr. Rhodes and his men helped support the rancher Cliven Bundy in an armed standoff with federal agents in Nevada.

Mr. Van Tatenhove was also involved in the Oath Keepers’ media operation when the group deployed as self-appointed vigilantes to Ferguson, Mo., after chaotic protests broke out over the death of Michael Brown, a young Black man killed by the police.

In more recent years, Mr. Van Tatenhove, who lives in Colorado, has turned to art and journalism, starting a blog about local news and culture in his state. He did not respond to several messages and phone calls seeking comment on his appearance before the committee.

Since leaving the Oath Keepers, Mr. Van Tatenhove has occasionally spoken out about the threatening nature of the group, though his testimony on Tuesday will offer the biggest public stage yet for his criticism of Mr. Rhodes and his subordinates.

“I have these self-realization moments where I’m like, ‘I helped these guys out, I helped spread the message,’” he told The Denver Post in an interview in February. “And yeah, it was just with words, but I’ve got to try to do something to try and make up for that in my own life.”