PART of the former IBM complex in Spango Valley has been destroyed by a deliberate fire.
Passing motorists saw flames shooting through the roof and alerted the emergency services.
People were warned today to stay away from the scene because asbestos may have been released by the blaze.
The old reception area went up in flames at 10.30pm last night.
Thirty firefighters in six appliances - including a high-reach vehicle - from Greenock and Gourock stations battled the blaze for around four hours.
Firefighters were still there this morning to damp down the blaze.
A fire service spokesman said: "The fire was well developed when we got there."
Detective Inspector David Wagstaff of Greenock CID says the incident is being treated as wilful fire-raising.
He said: "The building is in a dangerous condition because it is now structurally unsafe, and it is believed there may be asbestos in it. No-one should go near it.
"We would like to hear from anyone who saw any suspicious people or vehicles there around the time of the fire."
DI Wagstaff added that the security firm looking after the site had now stepped up patrols.
Anyone with information should call the police non-emergency number 101.
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