Susan Powell's Sister: Police Responsible for Death of Boys

April 1, 2012— -- Newly unsealed court documents have convinced Denise Cox, the sister of Susan Powell, that police could have prevented the death of Susan's two children.

Susan Powell went missing in 2009. Within hours of her disappearance, police in West Valley City, Utah, honed in on Susan's husband Josh Powell as a lead suspect. When they failed to make an arrest, Powell eventually moved to Washington state along with the couple's two children to be with his family.

This February, Powell murdered the couple's two children and took his own life when he had the children on what was supposed to be a supervised visit.

However, the documents unsealed in Pierce County, Wash., Friday, reveal police in Utah had compiled far more evidence on Powell's involvement in his wife's possible murder than ever previously known.

Cox blames police for not acting to protect the children.

"Look what it did -- it killed my nephews, by not putting (Josh Powell) behind bars when he should have been," Cox told ABC News. "They didn't act on anything that they should've acted on."

The documents reveal that police found Susan Powell's blood "on the tile floor next to the carpet adjacent to the sofa" in the couple's home.

When police initially encountered the sofa soon after Susan's disappearance, "two fans were set up and blowing" onto it and the documents say it "appeared to have just been cleaned."

Cox says she is also deeply disturbed by something else she says police did not tell the family about: A folded letter they discovered inside Susan's safety deposit box.

The letter had been "stapled around the edges addressed to her family and friends specifically asking that it not be shown or given to her husband Joshua Powell, writing that she did not trust him," according to the court documents

Chillingly, the documents say that letter also said that if Susan Powell dies "it may not be an accident, even if it looks like one."

Cox says police should have used this type of evidence to make sure the Powell children were never left alone again with Josh Powell. She also feels the family should have been told about the letter, as it was addressed to them.

"They were foolish in keeping it from us," Cox said. "All the signs were there and the police didn't act on it."

Late Sunday, reacting in part to Denise Cox's interview which was initially broadcast on "Good Morning America," West Valley City Mayor Mike Winder told The Associated Press he stands behind his police department.

"It is easy to Monday morning quarterback these things. It is easy for those of us who watch TV to say, 'Hey, there is plenty of evidence,'" Winder said. "It is a different matter if you are in the legal community to have enough evidence to go in for an arrest."