Tens of thousands of Facebook users across the U.S. have been falsely “checking in” at the Standing Rock Indian Reservation in North Dakota after a viral message claimed that investigators were tracking protesters on Facebook.

“The Morton County Sheriff’s Department has been using Facebook check-ins to find out who is at Standing Rock in order to target them in attempts to disrupt the prayer camps,” said a version of the message, whose origin was unclear and which was collected by the hoax-busting site Snopes.

“SO Water Protectors are calling on EVERYONE to check-in at Standing Rock, ND to overwhelm and confuse them.”

But on Monday, the Morton County Sheriff’s Department said the claim was a hoax.

“In response to the latest rumor / false claim circulating on social media we have the following response,” the department said in a post on Facebook. “The Morton County Sheriff’s Department is not and does not follow Facebook check-ins for the protest camp or any location. This claim / rumor is absolutely false.”

Demonstrators have been stationed at Standing Rock for months to protest the planned path of a pipeline that they say could desecrate tribal lands and put their drinking water at risk.