New Jersey Sen. Cory Booker is joining an expanding list of members of Congress and civil rights advocates calling for a federal investigation into police tactics at the standing rock pipeline protest.

Booker is urging U.S. Attorney General Loretta Lynch to have the Justice Department intervene and send federal monitors to the scene. In a letter sent to Lynch on Friday, Booker is seeking to have the department investigate what he termed “credible reports of inappropriate police tactics” at the North Dakota protest site.

Booker "deeply troubled" over police response

“I am deeply troubled by this tense situation, and particularly by reports indicating that law enforcement may have been responding to peaceful protesters near Standing Rock with overly aggressive tactics,” Booker wrote in his letter to Lynch.

Booker was specifically referring to a violent confrontation late last Sunday when police used water cannons in below freezing weather to soak protesters, fired rubber bullets and lobbed tear gas canisters into the crowd.

The protesters -- who call themselves “water protectors” -- are trying to stop Dallas-based Energy Transfer Partners from building the final segment of Dakota Access Pipeline, a stretch of pipeline the protesters say could contaminate the water supply for the Standing Rock Sioux reservation.

Sheriff's officials say they were facing "an ongoing riot"

Morton County sheriff’s officials are defending the actions of their deputies and other officers on Sunday, claiming they were trying to fend off what they termed an “ongoing riot” by nearly 400 protesters.

But the demonstrators maintain their actions were peaceful and that police overreacted. Indeed, videos circulating over the internet show protesters standing or milling about, posing no apparent threat to heavily-armed police officers, while being doused by the water cannons. Protest leaders say nearly 200 people were hurt, some seriously.

The call by Booker for an investigation into police tactics comes as fellow Democrat, Vermont senator and former presidential hopeful, Bernie Sanders has been urging President Obama to step in to defend the protesters and to stop the pipeline.

Also on Thanksgiving Day, New Mexico Sen. Martin Heinrich issued a renewed request for the president to intervene and stop what he called “the recent escalation of violence” against the protesters.

The American Civil Liberties Union is also criticizing law enforcement at the scene, blasting police for using what the union terms “life-threatening crowd-control weapons” against the demonstrators. The ACLU has also been calling for a Justice Department investigation.

More than 2,000 veterans heading to protest site

Meanwhile, as the pressure is being ratcheted up for a federal intervention, more than 2,000 military veterans are heading to Standing Rock to join the protest. Organizers of the group calling themselves “Veterans Stand for Standing Rock” say they’re being flooded with support, with 2,100 veterans signing up to be part of what’s being calling a “peaceful, unarmed militia” while hundreds of thousands of dollars in donations to support their effort is pouring in.

The veterans plan to assemble at the protest site for three days, beginning Dec. 4. Through late Saturday, a gofundme site set up to help cover expenses for the group had raised nearly $400,000.

The arrival of the veterans on the scene is expected to complicate matters after the U.S. Army Corps of Engineers said it would be closing a stretch of federal land on Dec. 5 where hundreds of protesters have gathered for months.

The Army Corps says it wants the protesters to move from the area, but the protesters are vowing to stay.

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