A year ago Tuesday, Sophia Wilansky stood on a bridge just outside the Dakota Access Pipeline encampment when she was flattened by a deafening explosion.

She refused to look down at her left hand, because she could not feel it and feared it had been blown off.

It became the emblematic moment of violence in the 10-month standoff between authorities and protestors, and it is not known if the metal shrapnel that tore through Sophia’s forearm came from a cop’s concussion grenade or a protester’s propane-tank bomb.

Four surgeries later and still under FBI suspicion, the 22-year-old activist from Riverdale spoke publicly for the first time about the horror of Nov. 21, 2016.

It was 4 a.m. and the Williams College theater grad and lawyer’s daughter was on guard duty at the Backwater Bridge near Cannon Ball, North Dakota. It was dark, and she was bundled up in a puffer jacket against the 20-degree temperature.

She had volunteered, along with three others, to hold the ground in front of the span, which the protest army had cleared of police- and pipeline-company barricades earlier in the day.

Suddenly a cop on a loudspeaker yelled, “Get away!”

Instantly she was hit by three rubber bullets, in the groin, chest and left arm. She fumbled to pick up a plastic shield, but the onslaught escalated.

“I heard a loud blast and was knocked to the ground,” said Wilansky. “I was in complete shock.”

The explosion had ripped out the radius bone, muscle, nerves and arteries of her left arm, and her hand was hanging by a few threads of flesh.

Her comrades scooped her up, carried her to their car and drove about 30 minutes to a waiting ambulance at a local casino.

“It was the most painful thing I ever felt, but it didn’t make sense in that situation to freak out,” she said. “I just kept thinking about how I would soon be in a hospital with pain meds.”

With her free hand, she texted friends and posted on Facebook.

The post has since been taken down, and in the spring the FBI applied for a warrant to search her Facebook account as the feds sought evidence of Wilansky’s possible connection to homemade explosives, according to court documents unsealed last month.

She has not been charged, and her lawyer says the probe is baseless.

“It was intended to scare her and other [protesters] from speaking out about that incident,” said Lauren Regan, who heads the Civil Liberties Defense Center.

Wilansky says she plans to sue to procure the shrapnel and clothing removed by surgeons and collected by the feds, in order to use the evidence in legal actions she will pursue against the law enforcement agencies she believes hurt her that day.

The Morton County Sheriff’s Office denies using a concussion grenade or any explosive device that day.

“Those are lethal devices and those are not even something [the department] ever had or has in its inventory,” said Morton County spokeswoman Maxine Herr. She said police fired sponge and bean-bag rounds and one stinger ball, which launches dozens of tiny rubber balls and gas.

Police claim Wilansky and three others refused orders to emerge from behind a shield. Officers said they saw someone roll metal cylinders toward the protesters and three propane canisters were found where the explosion took place, Herr said.

Wilansky’s injury was the most severe to result from the protest led by the Standing Rock Sioux tribe, who feared the oil pipeline would pollute their drinking water and intrude on sacred grounds. The Obama administration temporarily halted the project, but it was completed under President Trump in June.

Wilansky, who can no longer use her left hand and has very little feeling in her arm, vows to continue the fight against climate change and for the rights of indigenous people.

“I will not let the threat of being injured or prosecuted deter me from standing up for what I believe in,” she declared.