Forensic investigators on Tuesday morning inspected the Provincial Waterworks Authority’s head office in Bangkok's Lak Si district after a fire there on Monday evening.
Engineering Institute of Thailand representatives will also inspect the structure.
PWA governor Seri Suparathit said a fact-finding committee would be set up to probe the fire.
Seri said police would speak to security guards at the office as part of the investigation.
The fire broke out at about 6.45pm on the fourth floor of the second four-storey building at the head office.
No one was hurt.
The blaze was brought under control in about two hours later and firefighters kept dousing it with water to prevent it restarting.
Police said the whole affected floor – comprising the office of PWA officials, the PWA-attached branch of the Office of the Auditor General of Thailand and a document storage area – was damaged and the roof caved in.