How the blaze took hold with such ferocity, killing at least 40 people and leaving many more critically injured, is now a key focus for officials.

The fire broke out at 1.30 a.m. (0030 GMT) in a bar called "Le Constellation", police said in a statement.

A 'fire of undetermined origin' broke out in a bar in the luxury Swiss ski resort town of Crans Montana, police said in a statement, adding that 'a major emergency response is…