Most people don’t remember this, or weren’t alive at the time, but the whole Colin Powell event at the UN was intended to stop the weapons inspectors.
France (remember the Freedom Fries?) wanted to allow the weapons inspectors to keep looking until they could find true evidence of WMDs. The US freaked out because France said it wasn’t going to support an invasion of Iraq, at least not yet, because the inspectors hadn’t found anything. That meant that the security council wasn’t going to approve the resolution, which meant that it was an unauthorized action, and arguably illegal. In fact, UN Secretary General Kofi Annan said it was illegal.
Following the passage of Resolution 1441, on 8 November 2002, weapons inspectors of the United Nations Monitoring, Verification and Inspection Commission returned to Iraq for the first time since being withdrawn by the United Nations. Whether Iraq actually had weapons of mass destruction or not was being investigated by Hans Blix, head of the commission, and Mohamed ElBaradei, head of the International Atomic Energy Agency. Inspectors remained in the country until they withdrew after being notified of the imminent invasion by the United States, Britain, and two other countries.
On February 5, 2003, the Secretary of State of the United States Colin Powell gave a PowerPoint presentation[1][2] to the United Nations Security Council. He explained the rationale for the Iraq War which would start on March 19, 2003 with the invasion of Iraq.
The whole point of Colin Powell burning all the credibility he’d built up over his entire career was to say “we don’t care that the UN weapons inspectors haven’t found anything, trust me, the WMDs are there, so we’re invading”. Whether or not he (or anybody else) truly thought there were WMDs is a bit of a non-issue. What matters was they were a useful pretext for the invasion. Initially, the US probably hoped that the weapons inspectors were going to find some, and that that would make it easy to justify the invasion. The fact that none had been found was a real problem.
In the end, we don’t know if it was a lie that the US expected to find WMDs in Iraq. Most of the evidence suggests that they actually thought there were WMDs there. But, the evidence also suggests that they were planning to invade regardless of whether or not there were WMDs.