Her husband, Josh Powell, who killed himself and their two sons last year, had been the primary person of interest in the case.
In announcing they were ending the active investigation, West Valley City police said Steve Powell, her father-in-law, "most likely" had no involvement in her disappearance, The Salt Lake Tribune reported Monday.
Powell is in prison in Washington state on unrelated charges.
Police and city officials released thousands of pages of document during the press conference that detailed many of the leads they had followed, all of which did not provide any clues to Susan Powell's fate or location.
Authorities collected lab results, photos, maps and even statements from psychics as the investigation spread to 11 states.
In one instance, they checked out a theory that Josh Powell had given his wife a poisoned pancake. An analysis of the meal found nothing unusual.
Investigators also wiretapped communications between Josh Powell and his brother Michael, whom police believed helped dispose of Susan's body. That yielded no clues, either, as investigators found the brothers used computer encryption to communicate. That encryption has yet to be cracked.
Michael Powell committed suicide in February.
Investigators spent several days searching for her remains in a rural area of Oregon.
Officials said they would look into any new leads that may be received.