@Walter Hinteler,
It's still not clear, if and how many people are dead/injured - 3 to 4 meters height of stones have to be searched.
A nearby grammar school (for more than 1,000 pupils) might be on risk to collapse as well.
The reason is, of course, not known yet. But it sems that the collapse might be due to works for a new underground line.
This is how the archive's building looked alike before: