Arwel Jones from the National Library of Wales told ITV News that he is hoping that the damage will be limited - "the most valuable stuff is housed in purpose-built cells at the very bottom of the library, and this fire is at the very top."
He admitted to concern that the damage from water pumped onto the building by firefighters would be as bad as the fire damage.
More top news
It will stay largely cloudy tonight and there will be some further rain around, with central parts perhaps staying driest.
ITV Wales understands the reason for the inquiry call stems from concerns raised at the end of 2016 over a former healthcare support worker.
Plaid Cymru says the Welsh Government must 'show stronger leadership' over fire safety at Welsh tower blocks.