Cara Delevingne's House Fire Perplexes L.A. Fire Dept.

Cara Delevingne 's devastating L.A. area house fire last month has left the LAFD scratching their heads ... despite their best efforts, they've officially ruled the cause undetermined.

LAFD officials tell TMZ the arson squad took on Cara's house fire investigation as standard procedure but couldn't pinpoint the exact origin and cause of the blaze due to the extensive damage.

With no witnesses and a lack of forensic evidence, investigators hit a dead end. However, they made it clear foul play was never suspected.

With no clear answers, the case has officially been closed.

Nonetheless, Cara's parents, Charles and Pandora , had their own theory about the house fire ... blaming a rogue powerline as the culprit, especially after a very windy night in SoCal.

CARA DELEVINGNE THROUGH THE YEARS

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We broke the story ... Cara's home went up in flames , sparking a massive response from the fire department, with the dramatic scene even caught on video.