The construction of the Dakota Access pipeline has begun. The full pipeline should be operational within three months, the developer of the long-delayed project said Thursday, even as an American Indian tribe filed a legal challenge to block the work and protect its water supply. The Army granted Energy Transfer Partners formal permission Wednesday to lay pipe under Lake Oahe, clearing the way for completion of the 1,200-mile, $3.8 billion pipeline. ETP spokeswoman Vicki Granado confirmed early Thursday that construction resumed "immediately after receiving the easement." Workers had already drilled entry and exit holes for the crossing, and oil had been put in the pipeline leading up to the lake in anticipation of finishing the project. "The estimate is 60 days to complete the drill and another 23 days to fill the line to Patoka," Granado said, referring to the shipping point in Illinois that is the pipeline's destination. Work was stalled for months due to opposition by the Standing Rock Sioux and Cheyenne River Sioux, as well as a prolonged court battle between the developer and the Army Corps of Engineers that oversees the federal land where the last segment of the pipeline is now being laid. President Trump last month instructed the Corps to advance pipeline construction. The Cheyenne River Sioux on Thursday asked a federal judge to stop the Lake Oahe work while a lawsuit filed earlier by the two tribes against the pipeline proceeds. Attorney Nicole Ducheneaux said in court documents that the pipeline "will desecrate the waters upon which Cheyenne River Sioux tribal members rely." ETP, which maintains the pipeline is safe, didn't immediately respond in court to the filing. U.S. District Judge James Boasberg didn't immediately rule. The tribes' lawsuit filed last summer maintains that the pipeline project threatens their water and cultural sites. It's been on hold while the river crossing dispute has played out. The Cheyenne River Sioux on Thursday told Boasberg that it also wants to make a claim on freedom-of-religion grounds. "The sanctity of these waters is a central tenet of their religion and the placement of the pipeline itself, apart from any rupture and oil spill, is a desecration of these waters," Ducheneaux wrote. Standing Rock Sioux attorney Jan Hasselman has said that tribe will also try to block the lake crossing in court, with likely arguments that further study is necessary to preserve tribal treaty rights and that it's part of the legal process for the company to obtain final permission to finish the pipeline. An assessment conducted last year determined the river crossing would not have a significant effect on the environment. However, the Army decided in December that further study was warranted to address tribal concerns. The Corps launched an environmental impact study on Jan. 18, but Trump signed an executive action six days later telling the Corps to allow the company to proceed with construction. Legal experts have disagreed on whether the Army can arbitrarily change its mind because of the change in White House administrations.