Denver Riggleman is a US air force veteran who became a one-term Republican congressman from Virginia. In the House from 2019, he was a member of the hardline Freedom Caucus and voted with Donald Trump more than 90% of the time. Yet according to his new book, Riggleman “began to understand that some of my colleagues had fully bought into even the more unhinged conspiracy theories” he had witnessed while campaigning.

In 2020, Riggleman lost his Republican nomination – after he officiated a same-sex wedding. In retaliation, someone tampered with the wheels of his truck, endangering the life of his daughter. “If I ever find the individual responsible, God help that person,” the former congressman writes now.

Out of office, Riggleman became a senior staffer to the House January 6 committee. Last spring, he resigned. The Breach is an account of what he learned, his decision to publish reportedly angering some on the panel.

The book is also a memoir, in which Riggleman describes growing up in a tumultuous home and his bouts with religion and his parents as well as the metamorphosis of the GOP into the party of Trump, and the events and people of January 6.

“The rift between Trump’s wing of the Republican party and objective reality didn’t begin with the election,” Riggleman writes.

He omits specific mention of birtherism, the Trump-fueled false contention that Barack Obama was not born a US citizen. He does acknowledge the “explosion of conspiracy theories during the Trump years”.

As a former intelligence officer and contractor, Riggleman places the blame on social media, algorithms and the religious divide. Together, such factors took a toll on the nation, democracy and the lives of the Republican base.

Hostility to Covid vaccines exacted an explosion in excess deaths among Republicans, 76% higher than for Democrats. In Florida, the Covid death rate eventually surpassed that of New York, to rank among the highest in the US. Owning the libs can kill you – literally. Tens of thousands died on Trump’s altar of Maga. For what?

Ron DeSantis, the Florida governor, actively encouraged vaccine skepticism. He refused to say whether he received the vaccine, and attempted to stop young children getting the shots. He is in the hunt for the Republican presidential nomination in two years’ time, second only to Trump.

The divine injunction against bearing false witness? It has elasticity.

Bob Good, a self-described “biblical conservative” who successfully challenged Riggleman for his Virginia seat, said Covid was a hoax. Jerry Falwell Jr, Good’s boss at Liberty University, left that fundamentalist powerhouse in August 2020, amid a scandal ensnaring him, his wife and a pool boy.

Falwell was also one of Trump’s most prominent supporters. Riggleman laments: “It was stunning to see true born-again holy rollers lining up behind Trump, a man who shunned church and had already been caught on camera bragging about grabbing women” by the crotch.

Likewise, he voices disgust for what has become of the party of Lincoln: “As a kid the people I knew respected a line between church and state. Trump’s party was veering more and more into Christian nationalism, where they demonized Democrats for having an unholy agenda.”

Riggleman is also horrified by the involvement of ex-servicemen in the Capitol attack. “There’s no denying it,” he writes. “The political challenge to the election was, at least on some level, linked to a military operation.”

He reserves some of his harshest criticism for those closest to Trump. Mark Meadows, his last chief of staff; Mike Flynn, his first national security adviser; Roger Stone, his longtime political aide. Each played a major role in the insurrection.

As Riggleman recounts, Meadows defied the committee and refused to appear for deposition. But he did turn over 2,319 texts and messages, avoiding prosecution for contempt of Congress. Some of those texts came from 39 House Republicans and five senators.

“Meadows gave us the keys to the kingdom,” Riggleman writes, also describing the Meadows texts as the committee’s “crown jewels”.

As for Stone, the Republican dirty trickster was an apparent link between the brains and brawn of the Capitol attack.

On 7 November 2020, hours after the networks called the election for Joe Biden, Stuart Rhodes, the founder of Oath Keepers militia, messaged: “What’s the plan … We need to roll.” Stone was part of the chat group. Rhodes now sits before a federal jury, charged with seditious conspiracy.

The final chapter of The Breach is devoted to Ginni Thomas, the wife of the supreme court justice Clarence Thomas. Its title: “The Better Half”. Riggleman raises Thomas’s past membership in Lifespring, a personal development program and purported cult. He says he “found Thomas in Mark Meadows’ text messages after a hot tip and a case of mistaken identity”. She wrote of “watermarked ballots” and a “military whitehat sting operation”. She mentioned “TRUMP STING w CIA director Steve Pieczenik” [actually a former state department official and conspiracy theorist]. She condemned “the Biden crime family and ballot fraud conspirators”.

Liz Cheney, the House committee vice-chair, asked Riggleman to pull back. The Wyoming Republican worried about exposing the Thomases as election deniers, QAnon followers, or both.

“I think we need to remove that briefing,” Cheney said, according to Riggleman’s telling. “It’s going to be a political sideshow.”

Months later, Cheney and the committee reversed course. On 29 September 2022, Thomas testified for more than four hours behind closed doors. She continued to claim the election was stolen.

In The Breach, Riggleman looks to the future.

“We have a new enemy in this country,” he writes, “a domestic extremist movement that is growing online at fiber-optic speed. Is there a road back? To be honest, I’m not quite sure.”

• The Breach: The Untold Story of the Investigation Into January 6th is published in the US by Macmillan