I would say it was more like "weapons of mass destruction that Saddam Hussein was HOPED TO possess". The Bush administration had a bit of circumstantial evidence that they sold to the world as definitive evidence, but they didn't actually know that there were no WMDs.
Correct me if my memory is failing, but they had several UN inspectors go to Iraq who found nothing and Tony Blair had his aides fabricate some false evidence that was later used by Powell to declare "hey, we found 'em, now let's nuk'em". When you need to fabricate evidence, you do it because you lack of actual one in the first place.
Saddam didn't let the UN inspectors look everywhere they wanted to, so that wasn't conclusive (in retrospect, this was an incredibly stupid thing for Saddam to do... maybe he wanted to bluff that maybe he did have WMDs to make himself appear more powerful than he actually was, not thinking that the US would actually invade?).
IIRC Powell's "evidence" (and some other evidence too) came from Curveball, not from the UK. It was very shaky evidence, but it wasn't known to be completely false. And even if it was, there was still the hope that Saddam had some kind of WMDs that we didn't know about.
I'm veering off into speculation now, but I think their thought process was... We gave him WMDs in the 80s and he used them in the 80s. What were the odds he really got rid of them all and hasn't tried to acquire more? I think everyone in the Bush administration just assumed that to be the case, so they weren't worried that the actual intelligence was incredibly suspect and possibly even fabricated. They already "knew" Saddam was guilty, they just needed to get the authority to take him out, and then they would be greeted as liberators and find copious evidence of all of Saddam's WMDs and evil deeds.
