"If I recall correctly, both the UK government and the US Senate conducted official investigations of this topic last year"
The UK investigation that you're refering to was the Butler Report. The terms of reference were set by the government and specifically did not include an investigation on whether the reasons for war were justified. Essentially it was a review of the WMD intelligence and the processes involved in collecting and presenting it. It was pretty damning against the UK intelligence processes and the government in terms of how they approached these.
The Butler Report can be found online if you're really interested.
Exactly, in contradiction to the first sentence of this article:
"...containing new evidence that the Bush administration manipulated intelligence to justify its plan to invade Iraq.."
We know via these official investigations the intelligence was crap and that our intelligence organizations are broken. This is a failure of our entire government (Executive failure and Congressional Oversight), not Bush or Blair 'manipulating intelligence'. What we need to worry about is that it is still broken.