Mmm... from a police site:
The 'bang' came from the detonator which it was originally thought had been defused, he said.
The explosive surrounding the detonator had already been made harmless, he added. But the detonator, which carried a very small amount of explosive material, was packed in a glass utensil.
"We are investigating as to how the detonator was brought to the court without being defused," Mr Ahmed said.
Full article: http://www.policeprofessional.com/news.aspx?id=25902