Federal prosecutors have charged 15 people with impeding the Trump administration's massive immigration crackdown in Minnesota earlier this year, accusing them of coordinating efforts to block arrests and deportations as part of a conspiracy against the U.S. government.

READ MORE: Minnesota county charges ICE officer in shooting during immigration crackdown

During a news conference Tuesday, Minnesota U.S. Attorney Daniel N. Rosen said the monthslong investigation had focused on members and associates of two activist groups that had "violently opposed the enforcement of federal law."

He characterized the groups as "antifa," an umbrella term for a diffuse movement of militant left-wing activists.

Twelve people were arrested Tuesday, two remained at large and one is already in custody, Rosen added. Information about their attorneys was not immediately available.

The arrests come as the Trump administration has escalated its crackdown on members of "antifa," which it labeled a domestic terror group and directed federal agencies to "investigate, disrupt, and dismantle" its affiliates and funders.

In March, eight people accused of having ties to antifa were convicted on terrorism charges in a Texas shooting, a first of its kind case that raised concerns among some civil liberties groups.

The 15 people charged Tuesday were part of "Direct Action Minnesota," a left-wing coalition of protest groups that played a role in the "surveillance, operational planning and rapid mobilization against law enforcement," Rosen said.

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Some had self-identified as "antifa," he said.

Their alleged actions included "stalking" U.S. Immigrations and Customs Enforcement agents, throwing blocks of ice at their vehicles and setting up blockades around federal buildings. Rosen declined to say whether any federal agents were injured as a result.

"Whether or not they actually, at the end of the day, cause bodily harm is not the measure of whether or not they committed a serious federal crime," he told reporters.

The defendants were each charged with conspiracy to impede or injury a federal officer, with some facing additional charges such as interstate stalking, destruction of government property and assault on a federal officer.

The alleged conspiracy began in January, shortly after the Trump administration launched its sweeping immigration crackdown, dubbed Operation Metro Surge, in response to reports of fraud within Minnesota's Somali community.

The crackdown brought thousands of federal agents into the Twin Cities and surrounding areas, often traveling in unmarked SUVs, at times banging down doors, waiting outside schools and demanding residents produce proof of citizenship. Federal prosecutors said the operation resulted in more than 4,000 arrests.

But the efforts were also stymied by a sprawling network of outraged Minnesotans, primarily organized through anonymous neighborhood messaging threads, that used whistles and car horns to call attention to the masked, heavily armed agents.

At the time, border czar Tom Homan indicated that federal authorities were probing "the organization and funding of the attacks on ICE."

"They'll be held accountable," he said. "Justice is coming."