A blaze at a former nursing home in Flanshaw was started deliberately, investigators have said.
Firefighters were called to the empty building in Batley Road yesterday (Friday) just after 4pm.
The central area of the complex was alight, with around a fifth of the building on fire.
Crews from Wakefield were joined by firefighters from Ossett, Dewsbury, Normanton and Leeds.
Five pumps, an aerial appliances and breathing apparatus were used.
Proposals were submitted last year to build a new Aldi supermarket on the site.
Today police confirmed a criminal investigation had begun.
Officers have already made house to house enquiries, but are appealing for anyone who witnessed suspicious behaviour outside the building or who has information on the fire to come forward.
Phone Wakefield District CID on 101.