Police and fire officers will team up to establish whether the fire which tore through the former Wesley building was started on purpose.
The fire in the former nightclub and church in Wesley Square began at around 8.30pm yesterday, with two appliances from Stranton and a hydraulic platform from Saltburn called in to fight the blaze.
A structural engineer has carried out an assessment of the building.
Cleveland Fire Brigade has now confirmed it has been "deemed safe to enter" with the inquiries between its officers and police ongoing.
Confirmation the fire was completely out was made at 12.30pm.
A spokeswoman for Cleveland Police added: "We are working with the fire service to determine whether the fire is suspicious or not."
She added officers will remain at the scene to ensure people keep a safe distance from the building, with the roads to remain closed until it is declared safe.
Anyone with information which could help police can call 101 or Crimestoppers anonymously on 0800 555 111.