The fire that has largely destroyed the derelict Marton Hotel and Country Club may have been started deliberately, according to Cleveland Fire Brigade.
Aerial photographs reveal the full extent of the damage.
Cleveland Fire Brigade said:
Crews brought the fire under control in the early hours of Friday morning, but smoke continued to billow from the building through the morning.
The hotel, which closed in 2017, has been gutted by flames, which collapsed large sections of the roof.
It took twelve fire fighting appliances, including an aerial platform, to bring the flames under control.
Cleveland Police said a number of people had already provided information and appealed for anyone who may have seen "suspicious people or vehicles" at the site to come forward.
They stressed that it could be two or three days before a thorough investigation of the site could begin.
In a separate statement released on Friday morning, Cleveland Police said:
Five surrounding homes were evacuated on Thursday evening and Cleveland Police urged locals to stay inside and avoid the area.
Evacuated residents have so far been unable to return to their homes.
Middlesbrough Mayor Andy Preston thanked the fire service for their work and said Middlesbrough Council would provide help to residents needing accommodation.