ITV News understands that there were people working on the roof of the National Library of Wales today, when the fire started.
Nobody is thought to have been injured.
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A survey into inappropriate behaviour questioned Assembly members and people who work either for individual AMs or the Assembly as a whole.
With breaks in the cloud this afternoon allowing the sunshine to shine through, pollen levels will rise to very high today.
It'll be humid and rather cloudy today, with some drizzle at times, mainly to the west of the mountains.