DHS official whose early warning about militias was rebuffed speaks out about Jan. 6

More than ten years ago, a report written by a Department of Homeland Security official warning about the resurgence of right-wing extremism caused outrage among conservatives and some veterans groups across the country.

Titled "Rightwing Extremism: Current Economic and Political Climate Fueling Resurgence in Radicalization and Recruitment," the report was met with harsh criticism from officials including then-House Minority Leader John Boehner, who accused the agency of not focusing on the "real threats," like Islamist terrorism.

The internal study singled out white supremacists and warned that veterans returning from Iraq and Afghanistan could be targeted for recruitment by right-wing extremists. But due to the ensuing political backlash, the DHS caved to pressure and dismantled the unit that had published the report -- and all work connected to analyzing and tracking the rise of violent right-wing extremism was halted.

Now, on the anniversary of the day that right-wing militia members helped storm the U.S. Capitol on Jan. 6, 2021, Daryl Johnson, the DHS official who wrote the report, says he and his team could "have repelled against the growth and the radicalization of [extremist] movements."

"We could've helped stem the tide and growth," Johnson told ABC News in an interview for the new ABC News documentary "Homegrown: Standoff to Rebellion," which charts the rise of the anti-government movements at the center of the Jan. 6 attack on the Capitol.

Johnson's study of extremist groups found that the mobilization toward violence began in 2008 after President Barack Obama was elected, as white supremacists and militia groups rose up alongside the organizations enraged by Obama's opposition to gun rights and his long-touted goal of reforming the nation's health care system.

Johnson said the formation of groups and movements like the Tea Party "serve as a breeding ground and a recruitment pool for more hardline extremists" like the Oath Keepers and Three Percenters, dozens of whose members were present on Jan.6.

"These two movements started getting more organized as their membership grew," said Johnson, "and they started actively participating in protest activity, organizing rallies."

Johnson said it was most disturbing to see the migration of these movements to social media, which he said allowed people to "radicalize, learn and train in the comfort and privacy of their own homes."

Johnson's DHS report, which was leaked to conservative media, was meant for law enforcement so they could "be prepared for the threat within," Johnson explained.

"And just like a disease or something of harm in our own bodies, this threat metastasized over time," said Johnson. "Year after year, the cancer and the virus spread within the body of America, to the point where we're now in a critical life-or-death situation."

In April, a Washington Post analysis of data compiled by the Center for Strategic and International Studies revealed a growing threat from homegrown terrorism, with right-wing extremist attacks and plots "greatly eclipsing those from the far left and causing more death."

According to the data, right-wing extremists have been involved in 267 plots or attacks, and 91 fatalities, since 2015.

Now more than ever, Johnson said, action should be taken to stop the growth of extremist groups.

"When you have these far-right groups active, you're going to have terrorist attacks," Johnson told ABC News. "We're in a time period where these groups are factionalizing; we see more violent elements come out of the splintering amongst different groups."

"A lot of people think the Capitol attack was the end of the show or the culmination of [extremist] growth and radicalizing activities," he said. "It's not."

ABC News' Josh Margolin, Jenny Wagnon Courts and Alex Hosenball contributed to this report.