The FBI has joined the search for a missing teacher and mother of three, investigating whether she was taken from Montana to another state against her will, an official tells FoxNews.com.
Sherry Arnold, a 43-year-old algebra teacher from Sidney, Mont., was last seen early Saturday when she set out for a run. A jogging shoe belonging to Arnold was found in a roadside ditch along one of her regular running routes, according to police.
Debbie Dujanovic Bertram, an FBI spokeswoman, told FoxNews.com that investigators are "looking into all angles" in the woman's disappearance -- including the possibility that Arnold was abducted and taken across the North Dakota border.
No solid evidence has emerged to indicate Arnold was kidnapped, authorities said, but FBI agents were called in to assist local law enforcement in the case. Officials would not elaborate.
"We're not ruling anything out at this point," Bertram said Tuesday.
Arnold left her home in Sidney for a run at about 6:30 a.m. Saturday. A witness reported seeing someone matching the missing woman's description that morning near where the shoe was found, Assistant Police Chief Robert Burnison said.
Sidney Mayor Bret Smelser said search teams Tuesday will re-canvass areas around town, looking for more clues to the woman's disappearance. After that, searches will be based on leads developed by local law enforcement and the FBI.
Snow and rain is expected Tuesday and temperatures are expected to plunge into the teens during the night.
Hundreds of volunteers have helped in the search for Arnold, a native of the town of about 5,000 along the North Dakota border.
School superintendent Daniel Farr said Arnold, who has three children, was a devoted algebra teacher who has taught in the Sidney system for 18 years. Her husband, Gary Arnold, works in the school district's administrative offices as director of federal programs.
"She's one of those teachers that every parent wants in front of their child," said Farr. "She's there early in the mornings and she's there after school. She is just a generous and caring person."
FoxNews.com's Cristina Corbin and the Associated Press contributed to this report.