"I'd like to find my daughter," Cox said. "I'd like to lay her to rest."
Police said both Steve and Michael Powell were uncooperative in the investigation.
They interviewed Michael numerous times after discovering he left his car at an Oregon junk yard weeks after Susan's disappearance — a fact police didn't learn until nearly two years later. Officials said he offered evasive answers about why he got rid of the car and how he had used it in late 2009.
His suicide left investigators without any person of interest in the case. While authorities believe the brothers were responsible for Susan Powell's disappearance, they said repeatedly Monday that they never had enough evidence to bring charges — an assertion that has been questioned in the past by legal experts as well as law enforcement in Washington state.
"This is a circumstantial case, yes," said West Valley City Deputy Chief Phil Quinlan. His fellow deputy chief, Mike Powell, added, "We didn't have a body. We don't have a crime scene."
The Powell brothers used sophisticated computer encryption to communicate, Quinlan said. Investigators have been unable to decipher that secret communication.
The case file shows that in August 2010, police contacted a West Valley City woman, whose full name is redacted, after her phone number was discovered in Josh Powell's phone records.
She told police she had a sexual relationship with Josh Powell after meeting him through a dating service about six or seven months before Susan Powell disappeared. The woman said she knew Josh Powell by the name John Staley, and she didn't know he was married. It wasn't until after she saw news coverage of the case that she discerned his true identity.
The woman called 911 just days after Susan Powell disappeared and claimed she had been having an affair with Josh Powell for the past two months, Maxwell wrote. At the time, however, she declined to provide corroborating information.
The file includes other details, and contained emails from Susan Powell's father, who expressed hope his daughter might be found in the days after her December 2009 disappearance.
Cox believed Josh Powell poisoned his wife's pancakes before she was taken from the couple's house.
Susan Powell was reported missing after failing to show up for work.
Police found a small amount of her blood on the floor next to a recently cleaned sofa and carpet in their house, but not enough to consider it a crime scene, they said.
Josh Powell eventually returned to the couple's hometown of Puyallup, Wash., where he got caught up in a battle with Susan Powell's parents for custody of the boys, 7-year-old Charlie and 5-year-old Braden.
On Feb. 5, 2012, he locked a social worker out of a rental home at the start of a supervised visit, attacked the boys with a hatchet and set the house afire. All three were killed.
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AP writer Paul Foy contributed from Salt Lake City, Utah.