The FBI says it believe Ms. Arnold is dead, however, authorities are still asking for help finding her body.
A missing Montana math teacher is believed to be dead, and the investigation has spread into western South Dakota.
Sherry Arnold disappeared last weekend. The mother of two went on a run and never returned home, but authorities did discover one of her shoes.
The FBI confirms one man in custody in North Dakota and another is being questioned in Rapid City in connection with her disappearance.
A special agent says a tip from the public led to the two men.
This article has been revised to reflect the following correction: