Detection dogs are set to be deployed across the site of a century-old derelict factory as investigations into the devastating inner-Sydney blaze continue.

The seven-storey former hat factory and a neighbouring building were destroyed in a catastrophic fire last month in Surry Hills.

The intense heat meant much of the brick and timber building collapsed and Fire and Rescue NSW crews have been carrying out emergency demolition works to make the site safe.

The derelict building was known to regularly house rough sleepers and 15 people were counted sleeping in the building on the night before the fire.

Despite efforts by police, two people who may have been in the building have not been located.

Cadaver dogs, set to search for possible human remains, will comb the site in coming days along with NSW Police investigators and specialist forensic officers.

The incident is being investigated by detectives from Financial Crimes Squad’s Arson Unit, under Strike Force Strontium, and police have spoken with four teenage boys over the blaze.

No charges have been laid.

– AAP