The US Army Corps of Engineers said on Sunday that it won't grant an easement for the Dakota Access oil pipeline in southern North Dakota.

The decision is a victory for the several thousand camped near the construction site, who've said for months that the four-state, $3.8 billion project would threaten a water source and cultural sites.

The pipeline is largely complete except for the now-blocked segment underneath Lake Oahe, a Missouri River reservoir. According to a news release, Assistant Secretary for Civil Works Jo-Ellen Darcy said her decision was based on the need to "explore alternate routes" for the pipeline's crossing.

"Although we have had continuing discussion and exchanges of new information with the Standing Rock Sioux and Dakota Access, it's clear that there's more work to do," Darcy said. "The best way to complete that work responsibly and expeditiously is to explore alternate routes for the pipeline crossing."