(CNN) -- A day after local police said they are ending their 3½-year search for a missing Utah woman, her father and his attorney told reporters that federal agents and private investigators are now on the case.
Susan Cox Powell disappeared from her home in West Valley, Utah, in December 2009. Her husband, Josh, killed their two sons and himself in an explosion in Puyallup, Washington, in February 2012.
Her father, Chuck Cox, said he won't give up looking for her, though he believes Josh Powell poisoned her with pancakes the day she vanished.
"My daughter's missing; she's probably not coming back," Cox told reporters in Seattle. "We're still going to look for her until we find her."
He said he understood why police in West Valley were ending their search due to the lack of a crime scene and body. He just wished they would have considered the circumstantial evidence more while Josh Powell and his brother, Michael, who also committed suicide, were alive.
A West Valley official said the case will remain open.
"We have no further active leads to develop at this point," City Manager Wayne Pyle said.
Cox's theory is that Josh Powell put something into the pancake batter he used to fix breakfast for the family. He said the husband served the pancakes individually and cleaned up thoroughly after serving the meal. Cox said it was uncharacteristic for Powell to be so helpful.
Susan Powell felt sick after eating the pancakes, Cox said.
Attorney Anne Bremner said she had learned that a federal agency -- she didn't know which one -- is investigating the case.
CNN affiliate KIRO said police released more than 30,000 pages of documents related to the case. One of those disclosed that Powell had had an affair with a woman the same year Susan Powell disappeared.
Another was a handwritten will from Susan Powell. The first page read: "For family, friend of Susan. All except Josh Powell, husband. I don't trust him!" It was dated June 28, 2008, KIRO reported.
Josh Powell maintained his innocence in his wife's disappearance until his death.
He had said he last saw her on December 6, 2009, after he and his two sons left after midnight to go camping in below-freezing weather in a desert area in Tooele County, Utah.
Bremner said her team, which now includes two private investigators, are going through the recently released documents and plan to search some areas where Cox believes his daughter may have been taken. Last week, searchers went to an area near Salem, Oregon, but found nothing.
CNN's Amanda Watts contributed to this report.