Crews were called out to Lower Horseley Fields at around 9pm on Monday which led to a number of roads being closed and railway lines being shut in the aftermath.
It saw a string of active businesses engulfed in the blaze with business owners speaking of the disruption it has caused to their livelihoods in the run-up to Christmas.
The fire was finally extinguished on Thursday after days of round-the-clock work by crews from across the West Midlands.
And now investigators from West Midlands Fire Service say they believe the fire was started deliberately – with crews set to remain at the scene for the next 24 hours to ensure there are no further flare-ups.
Lower Walsall Street will remain closed and Lower Horseley Fields will be one-way into the city, the fire service added.