President-elect Donald Trump holds stock in the company building the disputed Dakota Access oil pipeline, and pipeline opponents warn that Trump's investments could affect any decision he makes on the $3.8 billion project as president.

Trump's 2016 federal disclosure forms show he owned between $15,000 and $50,000 in stock in Texas-based Energy Transfer Partners. That's down from between $500,000 and $1 million a year earlier.

Trump also owns between $100,000 and $250,000 in Phillips 66, which has a one-quarter share of Dakota Access.

While Trump's stake in the pipeline company is modest compared with his other assets, ethics experts say it's among dozens of potential conflicts that could be resolved by placing his investments in a blind trust, a step Trump has resisted.

The Obama administration said this month it wants more study and tribal input before deciding whether to allow the partially built pipeline to cross under a Missouri River reservoir in North Dakota.

The 1,200-mile pipeline would carry oil across four states to a shipping point in Illinois. The project has been held up while the Army Corps of Engineers consults with the Standing Rock Sioux, who believe the project could harm the tribe's drinking water and Native American cultural sites.

The delay, which comes as protests unfold daily along the proposed route, raises the likelihood that a final decision will be made by Trump, a pipeline supporter who has vowed to "unleash" unfettered production of oil and gas. He takes office in January.

"Trump's investments in the pipeline business threaten to undercut faith in this process — which was already frayed — by interjecting his own financial well-being into a much bigger decision," said Sharon Buccino, director of the land and wildlife program at the Natural Resources Defense Council, an environmental group.

"This should be about the interests of the many, rather than giving the appearance of looking at the interests of a few — including Trump," Buccino said.

Trump, a billionaire who has never held public office, holds ownership stakes in more than 500 companies worldwide. He has said he plans to transfer control of his company to three of his adult children, but ethics experts have said conflicts could engulf the new administration if Trump does not liquidate his business holdings.

Rep. Raul Grijalva, D-Ariz., senior Democrat on the House Natural Resources Committee, called Trump's investment in the pipeline company "disturbing" and said it fits a pattern evident in Trump's transition team.

"You have climate (change) deniers, industry lobbyists and energy conglomerates involved in that process," Grijalva said. "The pipeline companies are gleeful. This is pay-to-play at its rawest."

Besides Trump, at least two possible candidates for energy secretary also could benefit from the pipeline. Oil billionaire Harold Hamm could ship oil from his company, Continental Resources, through the pipeline, while former Texas Gov. Rick Perry serves on the board of directors of Energy Transfer Partners.

Concern about Trump's possible conflicts comes as protests over the pipeline have intensified in recent weeks, with total arrests since August rising to 528. A clash this past week near the main protest camp in North Dakota left a police officer and several protesters injured.