A 22-year-old man suspected of killing his elderly grandmother and two others on Montana's Crow Reservation was arrested Wednesday night in Washington state after a daylong search, the FBI said.Sheldon Bernard Chase was arrested without incident in Spokane, Wash. He is suspected of using a rifle Tuesday afternoon to kill his grandmother, Gloria Sarah Goes Ahead Cummins, 80; his cousin, 21-year-old Levon Driftwood; and her boyfriend, 20-year-old Rueben Jefferson, the FBI said in a statement.The arrest follows a manhunt in which police, game wardens and tribal authorities combed county roads looking for Chase. Schools on reservations in southeastern Montana and western North Dakota were closed and residents stayed indoors for safety during the search.Eric Barnhart, FBI supervisor in Billings, told The Associated Press that investigators are still trying to piece together what led to the shooting."At this point, we still don't know," he said. "With some history of mental illness, that's an X factor."Barnhart declined to release any details about the circumstances of the arrest.The shootings happened at Cummins' home about 10 miles outside Lodge Grass, a town of 500 people near the Wyoming border. Chase lives with his uncle in a mobile home about 100 feet from the modest log house, city clerk Cody Not Afraid said.On Wednesday, police tape stretched across Cummins' property and Bureau of Indian Affairs law enforcement officers kept onlookers from approaching.It was unclear what led to the shootings or how Chase was identified as a suspect. Earlier Wednesday, Barnhart said that authorities considered Chase to be armed and extremely dangerous.People in Lodge Grass described the victims and the suspect as coming from a prominent, well-respected family in the community. Chase moved there about a year ago from North Dakota, where he had grown up, and was attending Little Bighorn College about 20 miles away in Crow Agency, said Rev. Jim Antoine, the priest at Our Lady of Loretto Catholic Church.