A little bit of both. It appears the intelligence community weren't as thorough as they could have been and presented intelligence that was undefinitive. However, the politicians were the ones who decided to interpret it the way they did and present it as definitive fact.
From the above linked article:
"In order to promote a war he had decided to fight with America come what may, the prime minister and his staff took intelligence that was sketchy and circumstantial and transformed it into something that appeared compelling and definitive. He can certainly argue that it was already faulty when it reached him. What he should not be allowed to do is evade responsibility for the way it was embellished once it reached his desk. Without this final step the case for war would have collapsed. "
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