THI-CHI: I think you're right about the timing of the announcement being political.
But whether the explosives went missing before or after the US invasion won't matter to the people who'll be blown up by them in the near future. What matters is how "targets" are selected by the groups that will use them, and how to stop them.
The US definitely needs to improve in this respect.
And in regards to the comment that these explosives were always available to be bought from other sources: Having such large amounts flood the blackmarket will surely make it more cheaper, so more easier, for groups to get their hands on them.