SAN FRANCISCO (CBS SF) — New details emerged Tuesday on the cause of the cracked window on San Francisco’s Millennium Tower.

A month after it was discovered, a report from building managers said an “exterior impact” is to blame for the crack.

However, officials said it is not clear yet what might have hit the window.

There had been speculation the crack could be related to the luxury tower’s sinking, but the report ruled that out.

The engineers recommended removing the safety scaffolding that was put up earlier this month to protect people from falling glass.