Video shows a bag sitting in front of the metal barriers right at the point of the explosion, in front of the most seriously injured people.
Then it shows a man literally jumping up after the blast and running at a dead run, while others are still apparently stunned. Of course, it might be normal to run. What is perhaps a little unusual is the speed and the fact that he is dressed in black.
The police had been had been asking about a man dressed in black who had been acting suspiciously, by some reports.
Still pictures of the bag and the aftermath of the explosion: