Powell was charged with 14-counts of voyeurism in Washington state.
He was convicted on all 14 charges.
"I feel Steve should answer for what he's done and should be off the streets," Chuck Cox told reporters following the verdict
Cox is the father of Susan Cox Powell who disappeared in 2009. Her husband, Josh Powell was the only person of interest in her disappearance. But he killed himself in February and also killed their two boys in a deadly house fire. The Cox family suspect Steven Powell may have been involved in Susan's disappearance.
"Now the only question is where is Susan?" Cox says. "Now maybe he'll answer it."
Cox hopes negotiations between the prosecution and the defense prior to sentencing will lead to information about Susan's whereabouts.
"These certainly open up the possibility for negotiations," he told reporters. "It's another step."
Other members of the Cox family were relieved that the jury after deliberating for six hours over two days came back with a guilty verdict on all fourteen counts.
"This is sweet justice," Denice Cox says. "He's been giving our family a hard blow ... and he got what he deserved."
Steven Powell's daughter, Alina Powell was crying after the verdict. She has been his most vocal supporter ever since Josh returned to Washington and moved in with them.
"My family was convicted two and a half years ago," she says. "I lost my sister-in-law, my sister, my brother, two darling nephews and now a great father. This has been a very complicated and difficult situation."
But back in Utah, Powell's other daughter was also relieved he was heading to prison. Jennifer Graves testified against her father during the trial.
"He's done some pretty terrible things," Graves told ABC 4 News. "I think that's where he belongs right now. And that means my father will not be on the streets and victimizing anyone else or bothering us either."
Powell was accused of video taping two young girls who lived next door in 2006. The charges stemmed from an investigation into the disappearance of Susan Powell.
Prosecutors told reporters that the average sentence for voyeurism in Washington state is between three and a half to five years. But they will recommend more time in prison than that.
Sentencing was scheduled for June 15.
Powell will also be a registered sex offender for the rest of his life.