GRAHAM, Wash. -- Investigators are trying to track down what started a fatal fire in Graham that killed three sisters, as a memorial for the sisters continues to grow.

People have left pumpkins, stuffed animals and flowers to remember the girls who died in a house fire.

One note reads, “My three granddaughters: You are in my prayers. I love you with all my heart.”

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Friends of the girls have been gathering to comfort each other, and school officials said crisis counselors will be on hand at two Pierce County schools that the girls attended to help students who want to talk about their loss.

Michaela Ballard, 10, and Emily Ballard, 11, both attended Pioneer Elementary in Spanaway. Their 13-year-old sister, Katelyn, attended Bethel Junior High School.

Fire officials said the three sisters were sleeping upstairs when the flames erupted.

The girls were spending the weekend with their father and stepmother, who both escaped the fire. The girls’ father tried to save them, but the flames were too much.

Sunday night, investigators brought in an accelerant-sniffing K-9 to search for signs of arson. Investigators said they are not ruling anything out.

Investigators removed some debris for analysis.

The investigation into the cause of the fire will continue Monday.