Assistant Chief Officer David McGown of the SFRS said it will scrutinise “every aspect” of the fire, which he said had left the Glasgow city centre building “devastated.”
Mr McGown said the service was determined to establish not only the cause of the fire, but how it spread, and what safety measures were in place at the building, considered the masterpiece of architect and designer, Charles Rennie Mackintosh.
Four appliances remained at the scene at midday today following the fire, which broke out in the historic Mackintosh Building at 11.19pm on Friday, before spreading to nearby buildings, including the O2 ABC.
A few pockets of fire remain, which crews were still working to extinguish.
The blaze is the second in four years to hit the building, which was undergoing a multi-million-pound restoration project to return it to its former glory.
Mr McGown said: “The investigation into this devastating incident has already commenced.
“Our fire investigation team has started gathering information and, working alongside our police partners, will look at various sources of information to establish exactly what has happened here.
“However, I would like to stress that this is a very complex process and will take time.”
He added: “Our fire investigations team are specialists in their own field.
“They will look at every aspect of this incident - including the cause of the fire, how it spread and what safety measures were in place.
“We understand the need for clarity here and why so many want to know how this happened, but I would again like to reiterate that we need to establish the full facts and circumstances of this incident - and this will take time.”
At the height of the incident more than 120 firefighters were involved in fighting the blaze. There have been no reports of any casualties.
The blaze has shocked and saddened people in Glasgow and beyond including leading figures in the worlds of art and music.
Singer Annie Lennox tweeted: “So sad and disappointed for everyone affected by this 2nd devastating fire... (who would believe??!!) at Glasgow’s historic and renowned Charles Rennie Mackintosh art school. .. the renovators, students, staff, alumni, noble fire fighters and everyone in Glasgow.”
Experts have estimated the cost of rebuilding the gutted Mackintosh Building would be at least £100 million, if anything can be salvaged at all.
However Glasgow School of Art officials said they remain hopeful of a positive outcome for the Mackintosh Building,
Meanwhile, Labour MP for Glasgow North East Paul Sweeney, who is director of the Glasgow Building Preservation Trust, said that having been escorted round the building by SFRS, it is clear the interior is almost entirely lost - but the main structural beams appear to still be largely intact, giving hope some of it can be saved.
Tweeting about his visit he wrote: “The Mack will endure.”