The FBI is investigating activists associated with the highly publicized Dakota Access Pipeline, according to reports published Friday.

Several agents within the FBI’s joint terrorism task force (JTTF) have tried talking to a handful of anti-DAPL activists, ostensibly about their actions opposing the line, according to a report published by The Guardian.

The agency has not revealed any reason for its repeated attempts at contacting the activists. Protests with the months-long pipeline delay have culminated in more than 600 arrests and episodic moments of violence.

Some attorneys believe the JTTF’s actions are unconstitutional.

Lauren Regan, the executive director of the Civil Liberties Defense Center, said there are three cases in which the FBI attempted “knocks and talks” allowing agents to show up at activists homes in hopes of securing voluntary interviews.

“The idea that the government would attempt to construe this indigenous-led non-violent movement into some kind of domestic terrorism investigation is unfathomable to me,” Regan said. “It’s outrageous, it’s unwarranted … and it’s unconstitutional.”

The activists did not respond to the agents, she noted.

Standing Rock Sioux, one of the American Indian tribes opposing the multi-state DAPL, has tried unsuccessfully to vacate activists from make-shift campuses since the U.S. Army Corps of Engineers rejected the $3.8 billion project in December.

Opposition to the line grew to a feverish pace after President Donald Trump signed a pair of executive orders in January approving the construction of the DAPL and Keystone XL. His order essentially wiped away the Army Corps’ decision.

Law enforcement agents at Morton County have continuously requested help from federal officials to quell what they call periodic violence at the campsites. More than 600 people have been arrested at the campsites over the past several months.

One report from December suggested that North Dakota’s former governor, Jack Dalrymple, asked Wisconsin for help dealing with “civil unrest” and “criminal activities related to opposition of the Dakota Access Pipeline (DAPL) project.”

The North Dakota Republican asked his Wyoming counterpart to send 40 officers to Morton County for assistance quelling what he deemed increasingly violent protests.

Dalrymple also wanted a 40/37 mm chemical munitions launcher, which could have been used to discharge tear gas on anti-DAPL demonstrators. The Morton County sheriff’s department used tear gas and high-pressured water hoses during November protests to disperse 400 “very aggressive” activists.

Agitators set nine vehicles ablaze, and destroyed construction equipment and debris on a bridge in October, resulting in the closure of Highway 1806. A woman was arrested in another incident after pulling a pistol from her waistband and firing shots at the police.

North Dakota officials believe that most of the protesters are from out of state. They are looking for evidence that environmentalist groups are paying the protesters to agitate against the DAPL, which is slated to transport nearly 500,000 barrels of Bakken oil from North Dakota to Illinois.

Tax Commissioner Ryan Rauschenberger, for instance, told reporters on Jan. 30 that his office is watching out for tax forms from various environmental groups opposing the project.

“It’s something we’re looking at. I can tell you I’ve had a number of conversations with legislators regarding this very issue,” Rauschenberger said. “[We’re] looking at the entities that have potential paid contractors here on their behalf doing work.”

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