"This is an amazing development," Anne Bremner said in an interview on FOX 13.
Bremner said the witness is a former co-worker of Steven Powell's, who has been in correspondence with her. She said the witness relayed to her what Powell had claimed.
Steven Powell has recently come under scrutiny about his alibi after Washington state authorities confirmed he abruptly called in sick for two days after Susan disappeared in 2009. He is currently in jail awaiting trial on voyeurism and child pornography-related charges, accused of taking inappropriate pictures of children and women.
Powell, through his lawyers, has refused to cooperate with West Valley City police in their investigation into Susan Cox Powell's disappearance. Police Chief Buzz Nielsen has suggested that Steven Powell and other members of Josh Powell's family could face criminal charges in connection with her disappearance.
Bremner said the witness claimed that Steve Powell described to her the trip, saying they "roasted marshmallows and sang songs." However, Steven Powell also told the witness that he had no knowledge of what went on during the weekend of Susan's disappearance, Bremner said.
"She went to the police a year ago, at least, and gave this information that was vital," Bremner told FOX 13.
West Valley City Police Sgt. Mike Powell said late Thursday he had no immediate comment on Bremner's claims.
"It doesn't surprise me if it's true, because I've suspected since the beginning that Steve was involved in her disappearance," Susan's best friend, Kiirsi Hellewell, told FOX 13.
Josh Powell killed himself and his children, Charlie and Braden, in an explosion and house fire on Feb. 5 inside a Graham, Wash., home. He lost custody of his children after Steven Powell was arrested when police served a search warrant on his home.
The family of Susan Cox Powell has said they are contemplating a lawsuit against police over their handling of the case.