A fire broke out at a showroom under construction in Mgr Carmelo Zammit street in Msida on Wednesday night.
The police were informed at around 2.45am and they found a number of bottles containing a flammable liquid on the roof. No one was injured.
Duty magistrate Audrey Demicoli appointed experts to help in the inquiry.
Meanwhile, the police said they had recovered a number of boxes of tools and CD pouches that had been reported stolen over the years.