BULLHEAD CITY, Ariz. (AP) — There is no evidence an Arizona house fire that killed five children was caused by arson, authorities said Wednesday.

Investigators with the Bureau of Alcohol, Tobacco, Firearms and Explosives found no trace of accelerants on debris samples taken from the December blaze, Bullhead City Police said in a news release.

The cause remains undetermined.

Both ATF and the Lake Havasu City Fire Department are still investigating.

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The father of four of the children left them and a young relative in the house on the afternoon of Dec. 16 while he went out to do some Christmas shopping.

Authorities haven’t released his name, nor will they say if he is facing any charges in the fire in Bullhead City, a close-knit community near the Colorado River and the Nevada border.

The blaze began in the downstairs foyer area of the two-story duplex, fire investigators said. Flames and smoke traveled up the only staircase inside the home, preventing the children from escaping. Their bodies were all found in an upstairs bedroom.

The victims were identified as 4-year-old Freya Jones and her three brothers — 2-year-old Elias Jones, 5-year-old Zane Jones and 13-year-old John Jones III — and 11-year-old Henry Lewis, a relative who was visiting.