The Army Corps of Engineers is halting construction of the Dakota Access Pipeline, the chairman of the Standing Rock Sioux Tribe announced on Sunday.

Most of the 1,172-mile-long pipeline has already been constructed. A small section that was slated to run under the Missouri River in southern North Dakota, located a mile from the Standing Rock reservation was not yet underway due to massive protests from residents and activists who worried about the pipeline's adverse environmental effects, proximity to the reservation, and location on stolen Native land.

Instead of allowing construction to continue, the Army Corps of Engineers will "be undertaking an environmental impact statement to look at possible alternative routes," chairman Dave Archambault II said in a statement.

"The Standing Rock Sioux Tribe and all of Indian Country will be forever grateful to the Obama administration for this historic decision," Archambault said. He also thanked the tribal youth who initiated the protests against the pipeline, which began in April, and the "millions of people around the globe" who expressed support for their cause.

"The Army's announcement underscores that tribal rights reserved in treaties and federal law, as well as Nation-to-Nation consultation with tribal leaders, are essential components of the analysis to be undertaken in the environmental impact statement going forward," U.S. Secretary of the Interior Sally Jewell said in a statement.

Since April, thousands of protesters from more than 300 indigenous tribes across the Americas, as well as non-Native demonstrators, have been camped out on the site. Beginning in September, contractors from a private security firm armed with mace and guard dogs began to attack protesters on the site. Various protesters have been arrested at the site, including Democracy Now! reporter Amy Goodman.

"With this decision, we look forward to being able to return home and spend the winter with our families and loved ones, many of whom have sacrificed as well," Archambault said. "We look forward to celebrating in wopila, in thanks, in the coming days. We hope that Kelcey Warren, Governor Dalrymple, and the incoming Trump administration respect this decision and understand the complex process that led us to this point."

Although protesters are celebrating the announcement, an attorney for the Standing Rock Sioux Tribe has warned that the Trump administration could overturn the decision.

Financial disclosure forms filed by President-elect Trump in May show that he owned between $15,000 and $50,000 in stock in Energy Transfer Partners, the company building the pipeline. Trump also listed $100,000 to $250,000 in stock in Phillips 66, which controls a quarter share of the pipeline.

Trump's stock in the pipeline is thus one of the many conflicts of interests his administration faces. Kelcy Warren, CEO of Energy Transfer Partners, donated $3,000 to Trump's campaign and an additional $100,000 to a Trump-supporting PAC and $66,800 to the Republican National Committee.

"They [Energy Transfer Partners] can sue, and Trump can try to overturn," attorney Jan Hasselman told the Guardian. "But overturning it would be subject to close scrutiny by a reviewing court, and we will be watching the new administrator closely."