More than 70 firefighters tackled a blaze in Pulborough which swept through a former steel factory.
It was feared the building was packed with Land Rovers being stored by a garage company but the boarded-up former Spirogills industrial unit in London Road was empty.
As an investigation into the cause was launched, a senior fire officer said the building may been used by rough sleepers.
The alarm was raised at 11.22pm last night. At the height of the blaze the brigade had ten fire engines and three specialist vehicles at the scene. It took 90 minutes to bring it under control.
Experts were still sifting through the debris today. Assistant Divisional Officer Roy Barraclough said: "The fire may have been started by someone who had been sleeping rough inside."
Roads around the area were closed. The last was reopened shortly before 9am.
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