Ongoing conflict between protestors at the Dakota Access Pipeline site and police turned violent Sunday night. A crowd of about 400 activists was met with sprayed water, tear gas, rubber bullets and flashbang grenades as they tried to breach a closed-off bridge near the construction site for the controversial pipeline.

The dispute over building the oil pipeline through Sioux lands has been simmering since April, and in October, law enforcement began attempting to clear the protestor camps south of the pipeline site. This resulted in the lone access point to the site, Backwater Bridge, becoming blocked by burned out vehicles and barriers.

Protestors began trying to clear the debris and cross the bridge on Sunday night, while setting fires in freezing weather. Authorities from the Morton County Sheriff's Department responded by bringing in a vehicle-mounted water cannon, as well as using non-lethal riot control measures.

Both the police and the protestors had different accounts of what happened that evening.

Indigenous Environmental Network organizer Dallas Goldtooth, a leader of the protests, told the Bismarck Tribune that the barricade was blocking access for emergency services to the Standing Rock Sioux Reservation. He claimed the protestors were legally on a public road, and trying to remove the vehicles to not only open the bridge, but to show the disproportionately militarized response of the police.

He also said the fires were set to help the group keep warm due to the low temperatures and use of the water cannon - though others watching a livestream of the protest claim the fires were started by tear gas grenades thrown by police.

But Morton County officials released a statement deeming the protest to be a "very aggressive" riot, and a sheriff's department spokeswoman told CNN that the fires were set before the water was sprayed, and that the water cannon was actually a fire truck attempting to control the spread of the fire. Authorities have also said that Backwater Bridge may not be safe for emergency vehicles to use because of damage caused during previous protests.

Whatever the cause of the escalation, the situation is still both delicate and dangerous. A local gym was opened up to protestors soaked by water during the incident, one officer was injured during the melee, and a number of protestors suffered injuries from rubber bullets - Goldtooth estimated as many as 180.

ATTN: reached out to the Morton County Sheriff's Department and to the Indigenous Environmental Network for comment, we will update this story if we hear back.