The straw was there to prevent shrapnel damage. Unfortunatel the straw ignited and started fires in a couple of surrounding buildings.

You can find a lot of pictures in german newspapers and magazines, i.e.: http://www.spiegel.de/fotostrecke/bombenfund-experten-spreng...

The did say that they found "Splitter" as far as 300m away, but that could apply to both shrapnel and just pieces of the casing.

Ah, I was thinking the straw/sand was more to dampen the shock-wave somehow. Interesting, I would have though that the shrapnel from a bomb that big would have been far more devastating even with sandbags; I guess they probably used more than I was picturing in my head. Fascinating stuff.