BILLINGS, Mont. -- Authorities resumed their search Tuesday for a missing Montana teacher who left her home for a pre-dawn run and never returned.
The only publicized clue into Sherry Arnold's Saturday disappearance is a single running shoe, found by a ditch along her running route in her hometown of Sidney.
The FBI and local law enforcement are investigating the possibility that Arnold, 43, was abducted from the town along the North Dakota border, which has been changing rapidly in recent years due to an oil boom.
After three days of searching by hundreds of residents, police, firefighters and others, the plan was to re-canvass areas around town that already had been searched at least once, officials said.
If nothing new in the case emerges, authorities said they would consider scaling back the search.
"At some point we feel like we've exhausted the possibilities in that area where she was last seen and apparently disappeared from," said FBI spokeswoman Deborah Bertrand.
Bertrand added that the search could take a different direction by Wednesday, potentially with searchers going to door-to-door in residential neighborhoods.
Arnold, a popular math teacher who grew up on a ranch outside Sidney, was married to another school system employee, Gary Arnold. The couple has five children from prior marriages, including two living at home and attending the same school where their mother worked for the past 18 years.
The school district has played an active role in the search by lending buses to transport members of search teams and setting up a fund to defer expenses.
Mayor Bret Smelser said the effort had covered all of Sidney and surrounding areas of Richland County. He said federal agents had met with family members, including Arnold's parents, Ron and Sherry Whited, and pledged to press hard for answers.
"The promise they made Ron and Sherry was that they wouldn't give up until they had found something or found Sherry," Smelser said.