Added Susan Powell's father, Chuck Cox: "[Steve Powell] knows a lot more than he's telling."
Bremner declined to say which agency was investigating.
The FBI in Salt Lake City declined to comment. The U.S. Attorney's Office for Utah said in a statement they "do not have plans to conduct any further investigation."
Susan Powell's husband, Josh Powell, 36, was the only named person of interest in the case until he killed himself and the couple's two sons in an explosion at his Washington state home on Feb. 5, 2012.
But Josh Powell's father, 63-year-old Steve Powell who is serving prison time in Washington state for voyeurism convictions was investigated by West Valley City police, who said Monday that he "most likely" had no involvement in the disappearance of his daughter-in-law.
Cox and Bremner expressed surprise that the released documents revealed that Josh Powell's brother, Michael Powell, also was a subject of the police investigation until his suicide in February in Minneapolis.
Cox noted that his daughter had felt Michael Powell "was one of the most normal people" in Steve Powell's family.
Cox now believes that Michael Powell was "deeply involved in this." Referring to his suicide, he conjectured that he may have killed himself out of "guilt, remorse or felt that police were closing in."
Cox said he also was surprised to learn from the released documents that Josh Powell apparently had an affair with a woman just two months prior to Susan Powell's disappearance.
According to a report dated Aug. 10, 2010, a woman named Kourtney told police she met Josh Powell through an online dating service, and claimed they had sex five to six times in various areas of the Salt Lake Valley. She said Josh Powell had paid her about $800 during this time frame.
"Kourtney mentioned that Josh would go by the name of John Staley," according to the report, and that he told her he had family, but they lived out of state.
Bremner said it was unclear if Kourtney was a prostitute, but that the liaison could have provided a motive for Josh Powell to kill his wife. Bremner noted there was also millions in insurance money at stake.
Susan Powell was last seen on Dec. 6, 2009, at her West Valley City home, which she shared with her husband, the only person of interest ever named in the case.
Josh Powell claimed he left his wife at home as he took his sons, Charlie and Braden, then ages 4 and 2, respectively, on a late-night winter camping trip in the West Desert of Utah.
He denied having anything to do with his wife's disappearance.
Josh Powell came under scrutiny early because he was the last one to see his wife alive and because he did not seem concerned about her disappearance. But police on Monday stopped short of saying Josh Powell killed his spouse.
"What we are saying at this point was Josh Powell was involved in the disappearance of his wife," said West Valley assistant Police Chief Mike Powell, who is no relation to the Powells in question.
Asked his reaction to West Valley City police closing the case, Cox said Tuesday he understood their reasoning, which included the lack of a body or crime scene, and no direct evidence pointing to Josh Powell or anyone else.
" .... but I personally think they came to the wrong conclusion," Cox said. "As far as the circumstantial evidence, they had plenty."
Bremner noted that in court, circumstantial evidence is every bit as valid as direct evidence. She added that if Josh Powell had been charged and arrested, his sons would likely be alive.
Salt Lake County District Attorney Sim Gill said Monday that West Valley City police and prosecutors had been working toward formally screening the case to determine what, if any, charges are appropriate when Josh Powell killed himself and his two sons last year.
But Gill noted that, along with other "deficits in putting these cases together," there was no indication of Josh Powell's mental state when Susan Powell vanished. "Even if we charge him, what do we charge him with?" Gill said.
Bremner said private investigators are now scouring the documents released by police to identify new areas to search for Susan Powell's body, searches that will likely entail the help of volunteers.
Cox said they would search "whereever West Valley City didn't search."
He said last week's search in Oregon originated with a phone call from someone in January. Cox said the call began the same way many others have, with the person saying they had contacted West Valley City, but police never got back to them. Cox said he persuaded police to mount the search by sending them an email.