BULLHEAD CITY, Ariz. (AP) — The cause of a house fire in northwestern Arizona that left five children dead last month is still unknown and could take months to determine, authorities said Tuesday.

Bullhead City police said the analysis of the fire debris is being conducted in a federal laboratory by chemists and electrical engineers with the Bureau of Alcohol, Tobacco, Firearms and Explosives.

The father of four of the children left them and a young relative at home for about 2 1/2 hours on Dec. 16 so he could buy Christmas gifts and groceries returned to find the charred remains of the family’s home.

Authorities haven’t released the man’s name and will not say if he is facing any charges in the fatal fire in a close-knit community near the Colorado River and the Nevada border.

Investigators said the fire began in the downstairs foyer area of the two-story duplex, and the flames and smoke traveled up the only staircase inside the home, preventing the children from escaping.

The bodies of the children were all found in an upstairs bedroom.

The Mohave County Medical Examiner’s Office announced Tuesday that 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 — plus 11-year-old Henry Lewis, a relative who was visiting.