Anne Bremner made the announcement at a Seattle news conference a day after local officials in Utah said they had closed their active investigation into the Susan Powell case.
"This is not over," Bremner said.
Newly released police records show that Utah officials believe Josh Powell likely killed his wife in late 2009, and that his brother, Michael Powell, helped dispose of the body, but authorities felt they didn't have enough evidence to prove that theory in court.
Last year, as the investigation continued, Josh Powell killed himself and his two young sons in an explosive house fire, leaving nearly all of his life insurance proceeds to his brother, Michael, who later jumped to his death from a parking garage in Minnesota.
Bremner, who was joined at the news conference by Chuck Cox, Susan Powell's father, said she and Cox were apprised earlier in the day of the federal investigation by an agent who has been directly involved in the case. She said she requested permission to announce the development at the news conference, and the agent granted it. Bremner said the scope involved looking into what Josh Powell's father, Steve, knew about his daughter-in-law's disappearance.
In response, Melodie Rydalch, a spokeswoman for the U.S. Attorney's Office in Utah, issued a statement saying that federal agencies in Utah had assisted in the investigation and would be happy to do so again should circumstances warrant.
"However, we do not have plans to conduct any further investigation," she said.
West Valley City Deputy Police Chief Mike Powell said he wasn't immediately aware of any ongoing federal investigation but would look into it.
Steve Powell had a sexual obsession with Susan Powell and thoroughly documented it in journals seized by police. He is currently serving a prison sentence after being convicted of voyeurism charges for secretly recording young neighbor girls.
Utah police said Monday that they do not believe he was directly involved with Susan Powell's disappearance but may know more about it than he has let on.
Both Bremner and Cox disputed the notion that prosecutors in Utah never had enough evidence to prosecute Josh Powell.
They cite his bizarre alibi -- that he wasn't home when his wife vanished because he had taken their two sons, then 2 and 4, camping in the Utah desert in the middle of a snowstorm.
They cite his unusual behavior -- that he showed little concern for her, couldn't explain why he had her cellphone with the digital SIM card removed, and, two days after she disappeared, he rented a car and drove it 800 miles.
They also cite potential motives: Josh Powell cleaned out Susan Powell's retirement accounts 10 days after her disappearance, and -- as the newly released documents revealed -- he had apparently had an affair with a woman he met through a dating service months before his wife vanished.
Cox said the police "came to the wrong conclusion on the circumstantial evidence. I think there was plenty."
Utah authorities have repeatedly said they didn't prosecute Powell because they did not have a body or a crime scene. While that makes it tougher to prove a murder charge, prosecutors across the country have won convictions in such circumstances.
Susan Powell's family plans to continue searching for her, with the help of volunteers and a private investigator.
"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, 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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