The FBI on Sunday asked landowners in parts of Montana and North Dakota to check for possible burial sites in a search for a missing teacher and authorities identified two men they are holding in connection with her disappearance.
Lester Vann Waters, 47, and Michael Keith Spell, 22, of Parachute, Colorado, were detained after a tip to a hotline set up by authorities, police said. They are charged with aggravated kidnapping.
The men were being held in a North Dakota county jail about 45 minutes from Sidney, Montana, where high school math instructor Sherry Arnold disappeared more than a week ago.
The FBI on Sunday said in a statement that Arnold may be dead but her body has not been recovered.
The FBI asked landowners in the northeastern corner of Montana and in three rural counties of northwestern North Dakota to report disturbed soil along tree rows -- or shelter belts -- because Arnold "may be buried in a shelter belt that contains mature or rotted trees," according to the statement.
Arnold, 43, was last seen on January 7 setting off for a predawn run. Her husband reported her missing when she did not return home.
Searchers found one of her running shoes on the outskirts of Sidney, which has undergone rapid growth amid a regional oil and gas boom.
Arnold's husband, Gary Arnold, expressed gratitude to those who helped him and his family, thanking them "for the love they showed us."
He told Reuters, "While this did not turn out the way we all had hoped, at least we are moving toward a resolution and an answer."
Waters and Spell will be extradited to Montana from the Williams County Law Enforcement Center, where they were being held, Williams County Sheriff's Deputy Steve Thompson said.
Bret Smelser, the mayor of Sidney, a town of 5,000 people on the upper Missouri River, said stepped-up oil and gas production from hydraulic fracturing has brought more people and economic activity to the community, as well as crime.
Firearms sales and permits to carry concealed handguns were on the rise in the aftermath of Arnold's suspected abduction, Smelser said.
"Before this, we always presumed we were safe and felt secure," he said.
The FBI said the search for Arnold's remains centers on far northeastern Montana near Sidney and Williams, McKenzie and Mountrail counties in North Dakota.
FBI spokeswoman Deborah Bertram said investigators are not seeking volunteers for a search and cautioned property owners inspecting shelter belts "not to put themselves at risk."
Aggravated kidnapping is a capital offense under Montana and federal laws.
(Editing by Karen Brooks, Ellen Wulfhorst and David Bailey)