A search is under way between Glendive and Billings for a 34-year-old woman last seen early Saturday in Glendive, while a Billings man has been arrested on charges he violated a restraining order the woman had filed against him in 1998, officials said.

Dawson County Sheriff Craig Anderson said searchers began looking for Susan Marie Casey Saturday night.

"This community is real concerned about Susan's status, and we have a lot of people looking for her," Anderson said.

Anderson said dozens of volunteers have covered nearly 700 square miles of Dawson County since late Saturday night. A helicopter from the Department of Homeland Security flew over the county Sunday looking for the woman, Anderson said.

Several private aircraft have been used in the search, and volunteers on horseback, on all-terrain vehicles and on foot have been at work since Sunday. The Yellowstone River is being searched by boat, he said.

The investigation into Casey's disappearance led to the arrest Monday of Walter M. Larson, 37, of Billings for violating a restraining order Casey filed against him in Dawson County in November 1998.

Court records said investigators found 44 attempted phone calls between Larson's phone and Casey's cell phone from last Thursday through Saturday, in violation of the restraining order.

Larson posted a $5,000 bond Monday afternoon and was released from jail. He was ordered to appear in Justice Court in Dawson County within 10 days.

Anderson declined to comment on a possible connection between the criminal charge filed against Larson and Casey's disappearance. He also declined comment on information previously released by Glendive police that a silver minivan possibly connected with Casey's disappearance was found in Billings.

Casey is 5 feet, 3 inches tall, weighs 125 pounds and has brown hair. Authorities have said Casey was last seen wearing a white hooded sweat shirt with writing on the front and stripes down the sleeves, blue jeans and tennis shoes.

She has four children.