Sidney Police Department/Reuters/Landov
05/15/2012 AT 06:30 PM EDT
Two men pleaded not guilty Tuesday to the kidnapping and murder of Sherry Arnold
after prosecutors filed court documents with new alleged details of her brutal attack.
Arnold disappeared at dusk along a rural road near her hometown of Sidney, Mont., where she was a star high school math teacher and mom to two kids and three stepchildren.
Before the arraignment, prosecutors added murder charges that make Michael Keith Spell, 22, and Lester Van Waters Jr., 48, both of Parachute, Colo., eligible for the death penalty.
According to the new documents, a jailhouse informant named Michael Pruit says Spell told him that Waters instigated the attack because the men were "high on drugs and Waters wanted to have sex."
Pruit says Spell told him that Arnold jogged past him and said, "Hi."
"Spell said he turned around and speared Sherry in the back of the head and knocked her down," the documents say. "Spell said he choked her out ... pushed Sherry's face into some water and drowned her to make sure she was dead."
Spell has allegedly told authorities that it was Waters who killed Arnold, according to other court documents filed earlier by the prosecution.
The documents say the men bought a shovel at a North Dakota Wal-Mart, dug a hole, placed Arnold's body on her side in a fetal position and buried her.
More than two months later, Waters allegedly led FBI agents to her body
, according to the court documents. Waters told authorities on the scene that, "I didn't kill that lady," according to the documents.