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Ted S. Warren, Associated Press
Karen Livolsi , upper left, walks with her daughter Katie, 16, center, and friend Alex Ramirez, 17, right, as they view a growing memorial to Charlie and Braden Powell, Tuesday, Feb. 7, 2012, at Carson Elementary School in Puyallup, Wash., where Charlie attended school. Charlie and Braden were killed Sunday, along with their father, Josh Powell, in what police said appeared to be a deliberately set fire by Powell at a home in Graham, Wash.

SEATTLE — Police in Washington state who searched a storage unit rented by Josh Powell found a stained comforter that tested positive for blood in initial exams, according to documents filed in court Friday.

Pierce County detectives reported finding a gray-blue comforter with a stain. The search warrant documents say the comforter tested positive for blood. Investigators planned to perform further tests.

Powell killed himself and his two young sons in a gas-fueled inferno Sunday, and Pierce County authorities consider it an admission that he killed his wife.

Investigators had considered Josh Powell a person of interest since his wife, Susan Powell, disappeared in Utah in 2009. At the time, Powell said he took his two sons ice camping in subfreezing temperatures.

Pierce County Prosecutor Mark Lindquist said presumptive tests are conducted with a field kit and that a determination of blood won't be confirmed until the item is examined in the lab.

"Field tests are not infallible," he said.

Lindquist said he expects the finding will be something law enforcement will share with colleagues in Utah who have been investigating Susan Powell's disappearance.

The comforter was the only item police kept.

Investigators also found toys and kids' clothes in garbage bags, as well as framed pictures in the storage unit. Those items were given to Susan Powell's family.

Baker reported from Olympia, Wash.