WASHINGTON (Reuters) - A sheriff’s department in North Dakota is investigating possible charges against the U.S. Green Party presidential candidate, Jill Stein, for damaging equipment during protests on Tuesday over construction of an oil pipeline.

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Stein was part of a group protesting the Dakota Access pipeline and spray-painted construction equipment, the Morton County Sheriff’s Department said. Stein does not dispute the account.

Protesters, who say the project will damage burial sites considered sacred to a nearby Native American tribe and pollute the area’s drinking water, also attached themselves to bulldozers and broke a fence, the department said.

Stein, a 66-year-old doctor and environmental activist from Massachusetts, said in a statement that she had spray-painted the words “I approve this message” onto a bulldozer, a nod to the phrase American politicians append to their advertisements to comply with transparency laws.

No arrests were made, the department said, and the investigation was continuing. The department also posted a picture on its Facebook page that showed Stein spraying the bulldozer, calling it a “criminal act.”

The $3.7 billion, 1,100-mile (1,800-km) pipeline project would be the first to move crude oil from the Bakken shale, a vast oil formation in North Dakota, to refineries on the U.S. Gulf Coast. Energy Transfer Partners is leading a group of firms to build it.

The project sparked violent clashes during the weekend between tribe members and other protesters and security officers near the construction site, and the Standing Rock Sioux Tribe is seeking a court order to block construction.

The Green Party opposes continued investment in oil extraction because of its link to climate change.

“I hope the North Dakota authorities press charges against the real vandalism taking place at the Standing Rock Sioux reservation: the bulldozing of sacred burial sites and the unleashing of vicious attack dogs,” Stein said in her statement. She said that pipeline security officials had used dogs and pepper spray on protesters over the weekend.

A spokeswoman for Energy Transfer Partners did not respond to a request for comment.

A Reuters-Ipsos poll in late August showed Stein the first choice of 2 percent of voters, trailing Democrat Hillary Clinton, Republican Donald Trump and Libertarian Gary Johnson. The U.S. presidential election will be held on Nov. 8.

Stein was arrested for trespassing in Texas during her 2012 presidential campaign when she tried to take supplies to protesters opposed to the Keystone XL pipeline.