By Debbie Emery - Radar Reporter
Arnold, 43, of Sidney, Montana, had vanished around 4.30 a.m. on January 7 after going out for an early morning run. As RadarOnline.com previously reported, a detailed search carried out by the FBI and local police had so far uncovered just one clue to her disappearance, a single running shoe, despite extensive canine, air and horseback units covering the case.
Federal investigators announced on Friday that a suspect was in custody being questioned for the murder of the wife and mother.
"Based on a tip from the public, law enforcement took one adult male into custody, and another adult male is being questioned by police," FBI spokeswoman Debbie Dujanovic Bertram said in a statement, adding: "The investigation to determine what happened to Ms. Arnold continues."
The discovery of Arnold's body was announced on the Sidney Public School website by officials, who had been alerted of her death earlier that morning after her family had been informed.
Students were let out of school early to mourn the loss of the high school and middle school math teacher who had taught there since 1993, and additional counselors were provided to help handle the grief.
"Sherry is one of those teachers that every parent wants in front of their child," Sidney School District Superintendent Daniel Farr told ABCNews.com.
"She is caring, first and foremost, one that is there before and after school helping students. She gets to know her students. She becomes a mentor to many a student that goes on to become a math teacher."
Arnold left behind her husband Gary, two children and three step-children.