Authorities expanded their search Monday for a high school teacher and mother of three who's missing from an oil boom town in northeast Montana, after recovering only a single running shoe since she failed to return from a weekend run.

The search for algebra teacher Sherry Arnold, 43, focused on a 10-square-mile area north of the town of Sidney near the North Dakota border. That's in the general vicinity of the roadside ditch where Arnold's shoe was discovered Saturday along one of her running routes.

Her family confirmed the shoe was hers, said Assistant Police Chief Robert Burnison. No evidence has emerged indicating foul play and the case is being handled, for now, as a missing person report, he said.

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, Burnison said.

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."

Hundreds of people, most of them volunteers, turned out for the search over the weekend. Volunteers were told Sunday they would be contacted if more searchers were needed. But Burnison said about 50 showed up anyway Monday to help about 100 to 120 law enforcement officers, police and others involved in the effort.

Mayor Bret Smelser, who attends church with Arnold's parents, said the outpouring of volunteers showed Sidney has remained a tight-knit community despite the changes brought by a massive influx of oil field workers in recent years.

The boom in the nearby Bakken oil fields of eastern Montana and western North Dakota has swelled Sidney's population from about 5,000 people several years ago to more than 6,000 today, officials said.

Smelser said Arnold was "a daughter of the community" whose disappearance brought out the best in the town's residents.

"My big fear as mayor is that we'll lose our small town charm and personality with the second wave of oil, but this is the way Sidney has always been. It's an amazing community," he said.

"What we need right now is everybody's prayers," Smelser said, of Monday's search. "The window of opportunity is slipping on us."

Nearly 1,000 people turned out for Sunday's search, but authorities limited the number to a more manageable 300. In addition to law enforcement officers and National Guard troops, a helicopter, several airplanes and three dog teams also took part.

Burnison said Monday's efforts started with searchers on the ground. Aircraft and search dogs would be called on as needed, he said.