The United States Army Corps of Engineers on Sunday said it had turned down a permit for the controversial Dakota Access Pipeline project. “The Department of the Army will not approve an easement that would allow the proposed Dakota Access Pipeline to cross under Lake Oahe in North Dakota,” the statement released by Army said. Native Americans and climate change activists have been protesting against the undertaking for several months.

The Army’s Assistant Secretary for Civil Works Jo-Ellen Darcy said she based her decision on the need to explore alternative routes for the pipeline. “Tribal officials have expressed repeated concerns over the risk that a pipeline rupture or spill could pose to its water supply and treaty rights,” she said, according to the statement. “Although we have had continuing discussion and exchanges of new information with the Standing Rock Sioux and Dakota Access, it is clear that there is more work to do.”

For months, Native Americans and environmental activists have been protesting against the construction of the pipeline, saying it would contaminate their water source and destroy sacred sites. Tribal chief Dave Archambault, in a release, thanked the Obama administration for this historic decision, reported The Guardian. However, this victory could be short-lived as US President-elect Donald Trump had expressed his support for the project, according to Reuters.