"A more robust analysis of alternatives can be done and should be done ... before an easement is granted for the Dakota Access Pipeline to cross the Missouri River on Corps land," Darcy wrote.
The news has been cheered by Standing Rock Sioux Tribe members and their supporters who argue that the pipeline, if it were to rupture at the lake, would endanger the tribe's water supply. The tribe's reservation lies a half-mile south of the proposed crossing location.
But tribal leaders warn their fight isn't over, as the Army's statement does not rule out approval for the current plan in the future.
"We are asking our supporters to keep up the pressure, because while President Obama has granted us a victory today, that victory isn't guaranteed in the next administration," said Dallas Goldtooth, lead organizer for the Indigenous Environmental Network.
The pipeline's builders, Energy Transfer Partners and Sunoco Logistics Partners, said late Sunday that they still would press for the project's completion "without any additional rerouting in and around Lake Oahe."
The companies alleged the Army's move was an attempt to delay a decision on the crossing "until President Obama is out of office." The companies received permission from the Corps to cross the lake in July, but they need a second form of permission -- the easement from the Army under the Mineral Leasing Act.
"The White House's directive today to the Corps for further delay is just the latest in a series of overt and transparent political actions by an administration which has abandoned the rule of law in favor of currying favor with a narrow and extreme political constituency," the developers said in a news release.
Protesters 'ready to keep going'
The $3.7 billion pipeline project would move about 470,000 barrels of crude oil daily from two North Dakota oil fields to an existing crude oil market near Patoka, Illinois. The pipeline would stretch across 1,172 miles through four states.
The North Dakota oil fields are said to contain an estimated 7.4 billion barrels of undiscovered oil.
The Army's decision comes after protesters spent months camping out in the area. The tribe, besides voicing concerns about water, also has said the construction would cut through sacred land and destroy burial sites.
The protests were largely peaceful but sometimes devolved into chaos, as law officers fired rubber bullets and tear gas and sprayed water at some activists they say were unruly. Authorities say some protesters set fires, vandalized construction equipment and threw objects at officers.
North Dakota's governor had ordered protesters to leave their campsite by Monday, citing harsh weather conditions. The Army Corps of Engineers had warned that activists could be arrested if they hadn't left by Monday, but the agency later said it had no plans to forcibly remove those who stay.
Instead of backing away, the protesters have recently come out in full force, even inviting US military veterans
to join their already robust presence.
"I'm really happy that I'm here to witness it and celebrate with a lot of my elders and the youth, but I think that we also need to keep in mind that we need to be ready to keep going," protester Morning Star Angeline Chippewa-Freeland said.
Army: Look for alternative routes
The tribe sued the Army in an attempt to stop the crossing after the Army Corps of Engineers granted permits in July.
Darcy's decision Sunday comes three weeks after her office announced it was delaying a decision about whether to grant the easement amid protests from the tribe and its supporters.
Since that time, she wrote, representatives of the Army, the pipeline backers and tribal officials discussed additional measures that "could further reduce the risk of a spill or pipeline rupture," including pipeline safety enhancements.
These proposals, as well as alternative routes and a deeper examination of the risk of a spill, need to be discussed further, Darcy said.
She called for the drafting of an official environmental impact statement
, a months-long process that would require further study and allow the public to weigh in. She did not announce a timetable or propose alternate routes for the pipeline.
Her letter noted that one previously proposed alternative -- having the pipeline cross the Missouri River about 10 miles north of Bismarck, which itself is well north of Lake Oahe -- was eliminated early in the planning phase.
Pipeline supporters speak out
Energy Transfer Partners and Sunoco Logistics Partners said Sunday night that they believe the Army's decision was a politically motivated move triggered by the White House.
Since the studying proposed by Darcy would take months, it remains to be seen how Obama's departure from the presidency on January 20 will affect events.
Jason Miller, spokesman for the transition team of President-elect Donald Trump, said Monday that the pipeline is generally "something that we support construction of." The Trump administration will review the matter after the inauguration, Miller said.
House Speaker Paul Ryan tweeted
his criticism of the Army's announcement, calling the intervention "big-government decision-making at its worst."
"I look forward to putting this anti-energy presidency behind us," Ryan tweeted.
North Dakota's sole member in the House of Representatives, Rep. Kevin Cramer, a Republican, slammed Obama.
"It was becoming increasingly clear he was punting this issue down the road," Cramer wrote in a statement. "Today's unfortunate decision sends a very chilling signal to others who want to build infrastructure in this country."
Opponents ready for next fight
The Army's decision may be useful in a court challenge, according to Jan Hasselman, an Earthjustice staff attorney representing the Standing Rock Sioux tribe.
"If the incoming administration tries to undo this and jam the pipeline through despite the need for an analysis of alternatives, we will certainly be prepared to challenge that in court," he said. "It's not so simple for one government administration to simply reverse the decisions of the former one."
May Boeve, the executive director leading environmental action group 350.org, celebrated the decision but also sounded a warning against any future plans to reverse it.
"The fight against Dakota Access has fired up a resistance movement that is ready to take on any fossil fuel project the Trump administration tries to approve," she said. "On Dakota Access and every other pipeline: If he tries to build it, we will come."