Trump administration withdrew legal memo that found 'ample legal justification' to halt Dakota Access pipeline The legal opinion was withdrawn two days before an easement was approved.

 -- Two days before the Trump administration approved an easement for the Dakota Access pipeline to cross a reservoir near the Standing Rock Sioux Tribe reservation, the U.S. Department of the Interior withdrew a legal opinion that concluded there was “ample legal justification” to deny it.

The withdrawal of the opinion was revealed in court documents filed this week by U.S. Army Corps of Engineers, the same agency that requested the review late last year.

“A pattern is emerging with [the Trump] administration,” said Jan Hasselman, an attorney representing the Standing Rock Sioux Tribe. “They take good, thoughtful work and then just throw it in the trash and do whatever they want to do.”

The 35-page legal analysis of the pipeline’s potential environmental risks and its impact on treaty rights of the Standing Rock Sioux and other indigenous tribes was authored in December by then-Interior Department Solicitor Hilary C. Tompkins, an Obama appointee who was -- at the time -- the top lawyer in the department.

“The government-to-government relationship between the United States and the Tribes calls for enhanced engagement and sensitivity to the Tribes' concerns,” Tompkins wrote. “The Corps is accordingly justified should it choose to deny the proposed easement.”

Tompkins’ opinion was dated Dec. 4, the same day the Obama administration announced that it was denying an easement for the controversial crossing and initiating an environmental impact statement that would explore alternative routes for the pipeline. Tompkins did not respond to a request by ABC News to discuss her analysis or the decision made to withdraw it.

On his second weekday in office, President Donald Trump signed a memorandum that directed the Army Corps of Engineers to “review and approve” the pipeline in an expedited manner, to "the extent permitted by law, and as warranted, and with such conditions as are necessary or appropriate." “I believe that construction and operation of lawfully permitted pipeline infrastructure serve the national interest,” Trump wrote in the memo.

Two weeks later, the Corps issued the easement to Dakota Access and the environmental review was canceled.

The company behind the pipeline project now estimates that oil could be flowing in the pipeline as early as March 6.

The analysis by Tompkins includes a detailed review of the tribes’ hunting, fishing and water rights to Lake Oahe, the federally controlled reservoir where the final stretch of the pipeline is currently being installed, and concludes that the Corps “must consider the possible impacts” of the pipeline on those reserved rights.

“The Tompkins memo is potentially dispositive in the legal case,” Hasselman said. "It shows that the Army Corps [under the Obama administration] made the right decision by putting the brakes on this project until the Tribe’s treaty rights, and the risk of oil spills, was fully evaluated."

Tompkins’ opinion was particularly critical of the Corps’ decision to reject another potential route for the pipeline that would have placed it just north of Bismarck, North Dakota, in part because of the pipeline’s proximity to municipal water supply wells.

“The Standing Rock and Cheyenne River Sioux Reservations are the permanent and irreplaceable homelands for the Tribes,” Tompkins wrote. “Their core identity and livelihood depend upon their relationship to the land and environment -- unlike a resident of Bismarck, who could simply relocate if the [Dakota Access] pipeline fouled the municipal water supply, Tribal members do not have the luxury of moving away from an environmental disaster without also leaving their ancestral territory.”

Kelcy Warren, the CEO of Energy Transfer Partners, the company behind the project, has said that “concerns about the pipeline’s impact on local water supply are unfounded” and “multiple archaeological studies conducted with state historic preservation offices found no sacred items along the route.”

The decision to temporarily suspend Tompkins' legal opinion two days before the easement was approved was outlined in a Feb. 6 internal memorandum issued by K. Jack Haugrud, the acting secretary of the Department of the Interior. A spokeswoman for the department told ABC News today that the opinion was suspended so that it could be reviewed by the department.

The Standing Rock Sioux and Cheyenne River Sioux Tribes are continuing their legal challenges to the pipeline. A motion for a preliminary injunction will be heard on Monday in federal court in Washington, D.C.

The Corps has maintained, throughout the litigation, that it made a good faith effort to meaningfully consult with the tribes.

The tribes contend, however, that the Trump administration’s cancellation of the environmental review and its reversal of prior agency decisions are “baldly illegal.”

“Agencies can’t simply disregard their own findings, and ‘withdrawing’ the Tompkins memo doesn’t change that,” Hasselman said. “We have challenged the legality of the Trump administration reversal and we think we have a strong case.”