The FBI said on Sunday that it believes a Montana teacher is dead and authorities are holding two Colorado men in connection with her disappearance.In a widening search for the body of the teacher, Sherry Arnold, 43, of Sidney, Mont., the FBI on Sunday asked property owners in extreme northeastern Montana and three North Dakota counties -- Williams, McKenzie and Mountrail -- to check their land for disturbed ground where a body could be buried.Landowners were specifically asked to check vacant farmsteads and "shelter belts," a line of trees that shields soil from wind erosion."Based on investigative evidence gathered over the last few days, it is believed that Ms. Arnold may be deceased; her body has not been recovered," FBI spokeswoman Deborah Dujanovic Bertram in Salt Lake City said in statement. "Ms. Arnolds body may be buried in a shelter belt that contains mature or rotted trees."The FBI cautioned landowners to not to put themselves at risk.Anyone who discovers something unusual was asked not to disturb the site and to immediately contact the Williams County Law Enforcement Center at 701-774-9191.Bertram stressed that authorities are not requesting volunteers to join in a search.Williams County Sheriff's deputy Jon Garrison says 47-year-old Lester Vann Waters Jr. and 22-year-old Michael Keith Spell, both of Parachute, Colo., are in the Williams County Correctional Center in Williston, N.D., awaiting extradition to Montana. Garrison says the two men face aggravated kidnapping charges in Montana. The two were brought to the Williston jail Friday.The jail is about 46 miles from Sidney, where officials say Arnold disappeared Jan. 7 while on an early-morning run along a truck route on the edge of the oil boom town of about 5,500 residents.On Friday, FBI Special Agent in Charge David J. Johnson, of Salt Lake City, said a tip from the public led to the two Colorado men. One man was arrested in Williston and another was being questioned by police in Rapids City, S.D. At the time, authorities were not identifying the men. "Well, obviously, it's not over," Sidney Mayor Bret Smelser said when told of the FBI news release. "We're angry. We're frustrated. But our concern is with Sherry's family and friends. And whatever support we can give to get them through this, that's what we'll do." Sidney school officials posted a statement online Friday saying they learned of Arnold's death that day. The statement provided no details. In the days after Arnold disappeared, hundreds of residents, police, firefighters and others combed the town and surrounding countryside without success. The only clue that has been publicly released was that one of Arnold's shoes was found along her running route. Arnold and her husband, Gary Arnold, have five children combined from prior marriages. Two live at home and attend the same school system where Sherry Arnold worked for the past 18 years.