(AP) – Federal investigators have joined the search for the cause of a massive weekend fire that destroyed a Manhattan apartment complex under construction.

WIBW-TV reports  that Manhattan Fire Chief Jerry Snyder says authorities aren’t ready to say if the fire early Sunday is considered suspicious.

The 96-unit Strasser Village Apartments had been expected to open sometime next fall. The luxury complex had been valued at $7 million, and Snyder calls the fire the most expensive in Manhattan’s history.

The federal Bureau of Alcohol, Tobacco, Firearms and Explosives is taking part in the investigation. ATF Special Agent in Charge Jeff Fulton said Wednesday the agency has 35 to 40 people working on the case.

The complex is located near a retail development, where the roofs of several stores were damaged by flaming debris.