Three teenagers were seen at Earnley Concourse moments before flames ripped through the building, police have said.
Fire crews were sent to the derelict conference centre at about 1pm on Saturday (October 5) following reports of smoke issuing from the building.
The fire is now being treated as arson, police have said, as they appeal for more information about the youths.
Earnley Concourse fire: Shocking pictures show scale of destruction
A Sussex Police spokesman said: "A fire at a derelict building in Earnley is being treated as arson.
"Firefighters were called to the unoccupied Earnley Concourse shortly before 1pm on Saturday (October 4).
"Two, possibly three, male teenagers were seen on the site just before the fire was discovered. One was described as blond-haired, wearing a black top, grey bottoms and carrying a bag. The second was wearing a dark top.
Earnley Concourse fire ‘under control’
"Anyone with information about the fire or the youths seen on the site is asked to report online or call 101 quoting serial 660 of 05/10."
A spokesperson for West Sussex Fire and Rescue Service said: “We received a call shortly before 1pm on Saturday October 5 to reports of a fire in a derelict building in Earnley.
“Two crews from Chichester were first on the scene. Sussex Control Centre also mobilised an off-road vehicle from East Wittering, two crews from Bognor Regis and the Command Support Unit, along with crews from Littlehampton and Midhurst.
‘Smoke billowing’ as eight fire engines tackle large fire near Chichester
"Two Hampshire Fire and Rescue Service crews from Emsworth and Havant and the aerial ladder platform from Southsea were also sent to the scene of the fire, along with water carriers from Petworth and Fareham and the Incident Command Unit from Haywards Heath.
“Bringing the fire under control, firefighters were able to sectorise the incident and used one high pressure hose reel, two fog nails and four jets to tackle the fire alongside the Aerial Ladder Platform. Firefighters also used eight sets of breathing apparatus, two triple extension ladders and one roof ladder to gain access to the property.
“Crews from both services worked exceptionally well together to quickly control a severe fire in a complex building.
“The incident was scaled back with some officers remaining at the scene to dampen down and identify any hotspots.
“Crews from Worthing, Littlehampton, Arundel, Lancing and Selsey were mobilised to relieve crews later in the day and overnight and carry out a watching brief to ensure there were no remaining signs of fire.
“The final pump left the scene at 9.05am on Sunday 6 October.”