The familiar dome on the top of the building on Union Corner collapsed, with buildings burned out and the station closed.
On Monday the man who ran the vape shop, giving his name as Arslan, spoke out about what happened.
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He told the Scottish Sun: “I am in grief at whatever has happened.
“It’s a hurtful and disappointing accident.
“Whatever it is that happened, it shouldn’t be happening.
“I am so upset and I don’t want to talk about it.”
The Scottish Fire and Rescue Service announced on Monday that a multi-agency investigation will take place as to the causes of the fire.
A statement said: “The cause of a fire that started in the ground-floor of a commercial unit in Union Street, Glasgow, causing the four-story structure to partially collapse, will be investigated by a multi-agency team.
“However, the Scottish Fire and Rescue Service (SFRS) cannot at this early stage speculate on what happened.
“More than 200 firefighters in total have been involved in the effort to make the area safe since the alarm was raised at 3.46pm on Sunday 8 March.
“As of the morning of Monday 9 March, eight fire appliances and three high reach vehicles remain at the scene.
“At the height of the incident, 18 fire appliances and specialist resources, including a high-volume pump drawing water from the River Clyde, were deployed to the scene.
“There are no reported casualties, either members of the public or SFRS staff, however the fire has caused extensive damage to the building and the loss of the building’s dome.
“Crews are working to cool off hotspots within the stonework and assist structural engineers in assessing the stability of the remaining building.”
However, due to the destruction caused by the fire, the service admitted it may never be able to definitively establish the cause.
A Scottish Fire and Rescue Service spokesperson said: “This has been a significant incident in Glasgow city centre, which has involved hundreds of firefighters working tirelessly since yesterday afternoon.
“We have now scaled back and eight appliances, including three high-reach vehicles, remain in attendance.
“The initial response involved crews from Glasgow and Lanarkshire, however any incident on this level would need to draw on resources from across Scotland as we bring in relief crews to take over.
“We cannot speculate on the cause of the fire at this time. In line with fire investigation protocols, a multi-agency investigation will be conducted, however due to the significant damage that has occurred this may be hampered.”