(CBS/AP) SEATTLE - The lawyer for the family of Susan Powell, a missing Utah woman, said Tuesday there's an ongoing federal investigation into her disappearance.
Anne Bremner made the announcement at a Seattle news conference a day after local officials in Utah said they had closed their 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 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.
During the investigation, Josh Powell killed himself and his two young sons and then left all of his belongings to his brother, 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 the federal probe could also involve what Josh Powell's father knew about his daughter-in-law's disappearance. Steve Powell is currently serving a prison sentence after being convicted of voyeurism charges.
Bremner said Susan Powell's family plans to conduct searches on their own, including a property in Oregon where authorities recently said they looked for evidence but found nothing.
The police documents, released late Monday, gave the public a glimpse into what police knew of Susan Powell's 2009 disappearance. Among the revelations: Josh Powell likely had an affair months before his wife disappeared.
Also, Susan Powell wrote in her journal that she was afraid of her husband and worried he might kill her.
The West Valley City Police Department has come under criticism for not doing enough to bring Josh Powell to justice. Documents show that detectives doubted Josh Powell's bizarre story that he left his sleeping wife at home in the middle of the night to take the boys camping in the Utah desert during a snowstorm, and they questioned his odd behavior in the days after the disappearance.
The files discussed how detectives painstakingly followed up on tips they received from campers, hunters, prisoners and other law enforcement agencies and even psychics.
Following Josh Powell's death last year - and his decision to make his brother the main beneficiary of his life insurance policy - police focused more closely on Michael Powell.
Michael Powell, an ardent supporter of Josh Powell, killed himself Feb. 11.
He was interviewed numerous times last year after investigators determined 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 the weeks after her disappearance.
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 initially 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. Josh Powell maintained his innocence and said he had taken the couple's young boys on a midnight camping trip in freezing temperatures the night she was last seen.
Police found a small amount of her blood on the floor next to a recently cleaned sofa and carpet in their house, and Josh Powell's bizarre behavior following the disappearance left lingering questions.
When he arrived at his house following his purported camping trip, he had his wife's cellphone in his van, with the digital SIM card removed. He couldn't explain why he had her phone, police said.
Asked why he didn't tell his boss he wouldn't be coming into work that day, a Monday, Josh Powell said he thought it was Sunday.
The day after Susan Powell was reported missing, Josh Powell rented a car at the Salt Lake City airport. He returned it two days later, having driven it more than 800 miles.
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. 15, 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 in the blaze.