CANNON BALL, N.D. — The Army Corps of Engineers on Sunday denied permission for the Dakota Access pipeline to cross under a section of the Missouri River, handing at least a temporary victory to the Standing Rock Sioux Tribe and its supporters.

The decision came after months of protests by thousands of self-proclaimed “water protectors” — bolstered by the arrival of more than 2,000 U.S. military veterans — who have opposed the pipeline out of concern that it could rupture and contaminate the river, which they say provides drinking water to the tribe and 17 million other Americans.

The pipeline is being built by Energy Transfer Partners, whose chief executive, Kelcy Warren, has said the company would not be willing to explore alternative routes.

The pipeline was nearly completed, save for the proposed section that would have passed underneath Lake Oahe, a reservoir on the Missouri River. The Army Corps of Engineers denied a permit necessary for that section to be built.

The Corps said Sunday it would explore alternative routes and could initiate a more thorough and lengthy environmental analysis of potential risks posed by the $3.8 billion, 1,170-mile pipeline.

“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,” Jo-Ellen Darcy, the Army’s assistant secretary for civil works, said in a statement. “The best way to complete that work responsibly and expeditiously is to explore alternate routes for the pipeline crossing.”

In a related memo, Darcy expanded on the Corps’ explanation for denying the permit. She cited policy by the White House Council on Environmental Quality, which has said in the past that concerns about tribal resources and environmental justice should, in some cases, “heighten agency attention” to exploring alternative solutions for projects. She said the increased analysis she announced Sunday for the Dakota Access pipeline reflected such heightened attention.

Darcy also noted that some documents related to the planned pipeline, including studies examining the risk of an oil spill, had been withheld from the tribe in the past “because of security concerns and sensitivities.” The tribe has said repeatedly, including in court, that it was not properly consulted on the pipeline and that the pipeline’s risk had not been fully examined.

In recent months, thousands of people have set up camp on the edge of North Dakota’s Cannonball River just north of the Standing Rock Sioux Reservation, vowing to place themselves in the way of the pipeline to block its construction.

Native Americans from across the country have joined the cause, as have environmental groups and celebrities.

As the news broke Sunday, the main Seven Council Fires camp erupted in celebration. Pipeline opponents joined hands in a giant human chain around the camp. Victory and honor songs rang out as children, not entirely clear what the fuss was about, sledded down hillsides slick with a recent snow.

“Pretty darn good!” said Kandi Mossett of the Indigenous Environmental Network. “This is really big news. My immediate reaction is, this is a victory. But we must remain vigilant.”

Pipeline opponents are pushing for a full environmental impact statement, a process Mossett said would take three years and would effectively kill the project.

A range of environmental and indigenous rights groups celebrated the Army Corps of Engineers’ decision.

“Today, the voices of indigenous people were heard. The rights of a sovereign nation were respected,” Rhea Suh, president of the Natural Resources Defense Council, an environmental advocacy nonprofit, said in a statement. “The Standing Rock Sioux and the myriad of indigenous communities by their side remind us all of the power of individuals to stand up and stand together to demand environmental justice.”

Standing Rock Sioux Tribal Chairman David Archambault, who broke the news to gleeful throngs at the main camp, cautioned vigilance. Opponents worry that President-elect Donald Trump, who recently reiterated his support for the pipeline project, could reverse the Corps’ decision.

“A denial only gets us to Jan. 20,” Mossett said.

Supporters of the pipeline attacked the decision.

“This purely political decision flies in the face of common sense and the rule of law,” Craig Stevens, a spokesman for MAIN Coalition, an industry group, said in a statement. “Unfortunately, it’s not surprising that the president would, again, use executive fiat in an attempt to enhance his legacy among the extreme left. That the president continues to believe that he is above the law is simply un-American, and it is this arrogance that working-class Americans soundly rejected on Nov. 8. For millions of hard-working people across the heartland, Jan. 20 cannot come soon enough.”

Stevens added: “With President-elect Trump set to take office in … 47 days, we are hopeful that this is not the final word on the Dakota Access pipeline.”

The news broke as thousands of veterans continued to stream in from across the country. Hundreds of them stood in a snowy field near Cannon Ball, where Rep. Tulsi Gabbard, D-Hawaii, and tribal elders addressed them. Many wore fatigues; others carried American flags.

“I honor you on behalf of my family, on behalf of the families and communities on Standing Rock,” said tribal elder Phyllis Young. “We hold all of the veterans in very high regard, and we have many who suffer PTSD.”

Choking back tears, Young said that her own daughter, a former Army captain, committed suicide two years ago. “I know that it was conditions of war,” she said. “But we have honored. We have been good Americans. We have served courageously. We have done our duty.”

Addressing the veterans, some of whom had tears streaming down their faces, Young added: “We don’t want any of you to have flashbacks. And that is the purpose of our peaceful venture. So that you can take on our belief system of peace. We care about you. Care about your health. Your conditions. Your well-being, spiritually, emotionally, physically. I thank you for being here.”

As she spoke, more buses and cars were pulling up on the narrow paved road alongside the gathering.

“Our numbers are vast,” said Dex McLelland, 27, a former Army infantryman who led a team of North Carolina veterans in a black Ford van. “We mobilized 2,500 people in four days.”

McLelland, a disabled veteran who served in Afghanistan, arrived wearing his dog tags and camouflage pants.

“You can’t allow big money to come in and rip up the Constitution in the faces of America’s oldest citizens,” he said. “These are the original Americans. Don’t you think they’ve caught enough slack? About makes you cry when you think about it.”

By late afternoon, the thousands of Native Americans, environmental activists and veterans gathered in the Seven Council Fires camp were taking the time to celebrate.

In a statement, Archambault expressed “the utmost gratitude for the courage it took on the part of President Obama, the Army Corps, the Department of Justice and Department of the Interior to take steps to correct the course of history and to do the right thing … . The Standing Rock Tribe and all of Indian Country will be forever grateful to the Obama administration for this historic decision.”