Cox already knew his 28-year-old daughter, who lives in West Valley City, had vanished because Josh Powell's sister, Jennifer Graves, called him Monday with the news.
Cox said he can't imagine where his daughter could be.
"She hasn't talked to her sister or anybody and that's what is concerning," he said.
Susan Powell was last seen just after midnight Sunday before her husband and their two young children apparently left for a camping trip, Cox was told by Graves.
The West Valley City police has been looking for Powell since Monday morning when Graves reported the whole family missing. Neither of the Powells showed up for work that day and the children were not dropped off at day care, Capt. Tom McLachlan said. Police went to the home to search for the family, but found nobody there. They did find Susan Powell's phone and purse.
Josh Powell and the children, ages 2 and 4, returned home sometime around 5 p.m. Monday, Cox said. Police interviewed Josh Powell and the older child, but they haven't been able to help police locate Susan Powell.
"It's still officially a missing person case, but it has become suspicious," McLachlan said. "No one can tell us or is [willing to tell] us her whereabouts, and she has not contacted us or anyone else we know about," he said.
McLachlan said it was too early in the investigation to presume possible wrongdoing or determine if anyone is a suspect or person of interest in the case.
"Her home was the last place she was seen, so we served the [search] warrant to see if there was anything there that would shed any light," he said. "[Detectives] will be going over the items [seized in the search] and should know soon what, if anything, they may have."
More than two dozen of Susan Powell's friends and church members braved temperatures of around 10 degrees Thursday night to hold a candlelight vigil at West View Park and pray for their missing friend.
Susan Powell's best friend, Kiirsi Hellewell, said she organized the vigil to keep the spotlight on her missing friend.
Hellewell said she walked home from church with Susan Powell around noon Sunday and saw the whole family enjoying themselves at a church Christmas party the day before.
"We want to see her come home safely," Hellewell said.