The fire occurred over the weekend near the town of Reasnor, Iowa, and destroyed construction equipment for the $3.7 billion oil pipeline project, according to the Jasper County Sheriff's office.
It was near the site of a previous blaze allegedly set by protesters in August.
Dakota Access "experienced the intentional burning of construction equipment by unknown individuals", the company said.
"These illegal actions have resulted in millions of dollars in damage."
The
