The day President Donald J. Trump urged his supporters to “be there, will be wild!” at the Capitol on Jan. 6, 2021, Alex Jones spread the message to millions.

“This is the most important call to action on domestic soil since Paul Revere and his ride in 1776,” Mr. Jones, the Infowars broadcaster, said on his Dec. 19, 2020, show, which airs live online and on a network of radio stations. Mr. Jones, whose lies about the 2012 Sandy Hook Elementary School shooting fueled years of threats against the 26 victims’ families, urged his listeners to take action.

A little more than two weeks later, Mr. Jones joined his followers at the Capitol as a behind-the-scenes organizer — a crucial role in the riot that is under increasing scrutiny by congressional investigators.

It is part of a reckoning Mr. Jones faces on multiple fronts. He is still fighting a half-dozen defamation lawsuits filed by the targets of his false claims, including the relatives of 10 Sandy Hook victims. Late last year the Sandy Hook families won four default judgments against him after he for years resisted court orders, and in upcoming trials, juries will decide how much he must pay them.

For Jan. 6, Mr. Jones helped secure at least $650,000 from a Publix grocery-store heiress, Julie Fancelli, an Infowars fan, to underwrite Mr. Trump’s rally on the Ellipse the morning of the attack, $200,000 of which was deposited into one of Mr. Jones’s business accounts, the House committee investigating the Jan. 6 attack said. The night before the riot Mr. Jones was at the Willard Intercontinental Hotel in Washington, where Trump aides and allies had set up an outpost. He has longtime ties to at least a half-dozen people arrested after the riot, including the founder of the far-right Oath Keepers militia, Stewart Rhodes, still a regular guest on Infowars, and Joseph Biggs, a former Infowars employee and Proud Boys leader.

The House committee has subpoenaed Mr. Jones, and included a three-page list seeking his related communications and financial records. The panel is also seeking Mr. Jones’s communications with Mr. Trump, his family and anyone from the White House or Congress in the days before the riot. Questioned by the panel this year, Mr. Jones invoked his Fifth Amendment right against self-incrimination more than 100 times, and is trying to block the committee’s demand for records in court.

Whatever the outcome of the Jan. 6 investigation, Mr. Jones’s journey from Sandy Hook to the assault on the Capitol is a reflection of how conspiracy theories in the United States have metastasized and corroded public discourse in the digital age. A defender of President Vladimir V. Putin of Russia and a former regular on RT, the Kremlin-funded international television outlet, Mr. Jones espoused such extreme views of American democratic society — he has cast airport security screenings as a plot to usurp Americans’ freedoms — that in 2011 RT stopped inviting him on air.

But after Mr. Trump appeared live in an interview on Infowars’ website in December 2015, Mr. Jones traveled from the fringes to become part of a newly radicalized Republican Party. Infowars grossed more than $50 million annually during the Trump presidency by selling diet supplements, body armor and other products on its website, records filed in court indicate. During and after the Jan. 6 riot, Infowars promoted its merchandise alongside graphic videos, including footage by an Infowars cameraman of the shooting death of a pro-Trump rioter, Ashli Babbitt, by a Capitol Police officer during the attack.

Mr. Jones did not respond to messages seeking comment. His lawyer, Norm Pattis, said his client had done nothing wrong on Jan. 6.

“Over many years Infowars has become a go-to source for people deeply suspicious of the government, so it should come as no surprise that many of the attendees at the rally had passed through Infowars’ doors,” Mr. Pattis said. “But that doesn’t mean any of them are guilty of criminal conspiracy or misconduct.”

Dan Friesen, whose podcast, “Knowledge Fight,” explores Mr. Jones’s place in America’s conspiracist tradition, said that people should not be shocked by what happened on Jan. 6, given Mr. Jones’s history. “This kind of flare-up just seemed inevitable,” he said.

Mr. Jones owes some of his core conspiracy themes to Gary Allen, a speechwriter for the former Alabama governor George Wallace who in the 1960s and 1970s was one of the far-right John Birch Society’s most revered writers and thinkers. As a teenager, Mr. Jones found Mr. Allen’s 1971 “None Dare Call It Conspiracy” on his father’s bookshelf, and came to share Mr. Allen’s view that a cabal of global bankers and power brokers, not elected officials, controlled American policy. Mr. Allen, who died in 1986, sold his theories by mail order in books, filmstrips and cassettes, a marketing model later adopted by Infowars.

Mr. Jones got his start in broadcasting in the early 1990s with simultaneous shows on the Austin radio station KJFK and on Austin community access TV. In 1993, a siege by federal law enforcement ended in an inferno at the Branch Davidian compound near Waco, Texas, killing about 80 Davidians and four law enforcement officers. Mr. Jones asserted, evidence to the contrary, that the sect and its leader, David Koresh, were a peaceful religious community marked by the government for murder. He raised $93,000 from his listeners to rebuild the compound’s church.

The deed made Mr. Jones a celebrity among “patriot” militia members, including some involved in armed standoffs with the federal government. In 1995, Mr. Jones pushed bogus claims that the government plotted the bombing of the Alfred P. Murrah Federal Building in Oklahoma City that killed 168 people, including 19 children. The perpetrator, Timothy McVeigh, had also expressed rage at the Branch Davidian compound’s destruction.

Mr. Jones and his wife at the time, Kelly Jones, founded Infowars around 1999, when they began producing feature-length, conspiracy-themed videos that they sold by mail or gave away, urging people to pass them around and spread the word.

After December 2012, when Mr. Jones falsely claimed that the Sandy Hook shooting was a government pretext for draconian gun control measures, traffic to his website surged. In 2013, at a gathering in Dallas marking the 50th anniversary of John F. Kennedy’s assassination, Mr. Jones met Roger J. Stone Jr., a Trump friend and adviser shunned by mainstream Republicans.

Mr. Stone, who saw a valuable new constituency for Mr. Trump in Infowars’ disaffected audience, joined the show as a host and brokered Mr. Trump’s December 2015 interview with Mr. Jones. In that interview, broadcast on the Infowars website, Mr. Trump joined Mr. Jones in casting America as a nation besieged by “radical Muslims” and immigrants, and predicted he would “get along very well” with Mr. Putin. He ended by praising Mr. Jones’s “amazing reputation.”

The next year Mr. Jones was a V.I.P. invitee to Mr. Trump’s speech accepting the presidential nomination at the Republican National Convention in Cleveland, where the Infowars broadcaster stood on the convention floor with tears streaming down his face as Mr. Trump spoke.

The Trump era also brought Mr. Jones new scrutiny. In 2017, he dodged a lawsuit by publicly apologizing and removing from Infowars his shows promoting Pizzagate, the lie that top Democrats were trafficking children from Comet Ping Pong, a Washington pizzeria. The conspiracy theory inspired a gunman to enter the restaurant and fire a rifle inside. No one was hurt, but the episode shocked the capital and many Americans. By 2019, Mr. Jones had been barred from all major social media platforms for violating rules banning hate speech.

Mr. Jones’s lawyers have downplayed his influence. “He’s playing a character,” one of the lawyers, Randall Wilhite, told a Texas judge in 2017.

By late 2020, when Mr. Jones was spreading the word to his listeners about Jan. 6, Mr. Rhodes made a pivotal appearance on the Infowars website. “We have men already stationed outside D.C. as a nuclear option in case they attempt to remove the president illegally,” Mr. Rhodes told Mr. Jones on Nov. 12, 2020, five days after President Biden was declared the winner. Mr. Rhodes pledged to provide security for a pro-Trump rally in Washington that occurred two days later.

Mr. Jones responded: “Stewart, can you not feel history happening right now?”

(Mr. Jones later told his audience that he thought the Oath Keepers’ plans for Jan. 6 amounted to “playing soldier” and that if the group had indeed planned to “detonate a rebellion,” he did not support that.)

But on the night before the Capitol attack, when Mr. Jones was with Trump allies at the Willard Hotel, he told a cheering crowd at a nearby rally that “globalists” had tried to steal the election. “As I told them 20 years ago, I’ll tell them again: I don’t know how this is going to end, but if they want a fight, they’d better believe they’ve got one!” he said.

The next day Mr. Jones marched with Ali Alexander, a “Stop the Steal” organizer also summoned by the Jan. 6 committee, from Mr. Trump’s speech on the Ellipse to the Capitol grounds.

In trials beginning this spring, juries will determine how much Mr. Jones must pay the Sandy Hook families in damages. Although the damages could sink Mr. Jones’s business, some of his recent comments suggest that it is the committee’s investigation that worries him more.

“Believe me, the problems with this is the least of my problems,” Mr. Jones told Mark Bankston, a lawyer for the families of two Sandy Hook victims, in a December deposition.

“You’re referring to Jan. 6?” Mr. Bankston asked.

“All of that, yeah,” Mr. Jones replied.

This article is adapted from “Sandy Hook: An American Tragedy and the Battle for Truth,” to be published by Dutton on March 8.