A 51-year-old woman from Baħrija sustained life-threatening injuries when she fell into a shaft, about a storey high, in a building that is still under construction in Tarxien.
The incident happened on Sunday night during celebrations of the feast of our Lady of Annunciation, the police said.
Members of the Civil Protection Department rescued the woman from the shaft.
She was rushed to Mater Dei Hospital where it was certified that she was in a critical condition.
Magistrate Silvio Meli appointed experts to assist him in an inquiry.