The man was in an office at the demolition site of the ex-Odeon cinema in Newcastle, at 07:40 BST on Tuesday, hours after the cinema collapsed.
Scaffolding and part of the building in the process of being demolished fell on the road on Monday night.
The Health and Safety Executive (HSE) said it was probing allegations relating to alcohol.
A spokesman also said it was investigating whether correct working practices were in place during the work.
The 1930s-built cinema, was in the process of being demolished when part of it came down shortly after 23:00 BST on Monday.
A bus shelter was crushed by scaffolding and falling masonry, but no-one was hurt in the collapse.
Demolition work on the cinema began earlier this year.
Claims that a workman was seen drinking alcohol at the site of a collapsed cinema are being investigated.