HELENA, Mont. -- U.S. Bureau of Indian Affairs officers escorted two children off an American Indian reservation in Montana where their grandmother brought them amid a custody dispute, bringing the federal government into a clash between state and tribal courts.

One officer detained Patsy Fercho on the Northern Cheyenne Indian Reservation on Nov. 21 while another drove the boys to the reservation border and turned them over to their father, serving a warrant issued by a court in Minnesota, where the father lives, attorney Roberta Cross Guns said Tuesday.

Fercho had picked up the boys in Minnesota in late September and took refuge on the reservation, believing she was out of reach of state court orders and protected by a tribal court order that gave her custody.

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"This is one of those areas where the BIA acts independently," said Maylinn Smith, associate professor and co-director of the Indian Law Clinic at the University of Montana. The bureau is "supposed to be operating on behalf of the tribe, but they aren't under their jurisdiction. This is a perpetual problem with BIA involvement in tribal affairs."

Bureau officers, who are federal employees, are required to recognize state court orders but not all tribal court orders, Smith said. An Indian Affairs spokeswoman did not return a phone call or email Tuesday seeking comment.

There is a process in which the tribe could complain to the bureau's local office that the officers' actions were not consistent with their obligations to the tribe, Smith said, but there's no statute that says an Indian Affairs officer can't execute a state court order on the reservation.

A Montana judge had ordered the boys' father to have temporary custody in late 2014 because their mother was unable to care for them. The mother, Fercho's adopted daughter and a tribal member, was battling addiction and legal problems.

Fercho asked the boys' tribe to intervene while also seeking custody through Minnesota courts. The Northern Cheyenne tribe ordered emergency guardianship and physical custody of the children to Fercho this fall after finding they were in danger of bodily injury.

The boys' father has not been charged with any abuse. His attorney, Rich Batterman, has declined to comment, saying it was a confidential child custody case.

A Minnesota judge found Fercho and her husband in contempt of court Nov. 20 for failing to follow a court order to return the children to their father and issued warrants for their arrest. That's when the Bureau of Indian Affairs got involved.

"It disturbs me that it was so coordinated," said Cross Guns, Fercho's attorney. The bureau "didn't consult with the tribe, but they did coordinate with a non-Indian guy to a great extent."

A federal court might end up deciding who has jurisdiction, taking into account the Northern Cheyenne tribal code, Smith said.

Tribal President Llevando Fisher did not return a phone message from The Associated Press seeking comment.

Cross Guns said she's working to see if the tribe will take a stronger position in the case and is asking the Minnesota court to quash the arrest warrants because the boys have been returned to their father.