No, actually they didn't, but the Wikileaks release of nearly 400,000 documents from the Iraq War shows that there were indeed many cases of American troops discovering biological and chemical warfare shells. Only problem of course, is that all of the shells discovered were old, in small numbers and in poor repair, suggesting that the Saddam Hussein regime was sloppy about disposing of weapons that the UN had ordered them to dispose of. What the shells do not suggest is that there was a working program to launch chemical or biological warfare against even close neighbors, let alone against the US.
Britain's The Guardian publishes extensive sets of documents concerning the Wikileaks release
Huffington Post summarizes many finding
Al Jazeera's nearly hour-long video includes a Pentagon spokesperson and Wikileaks' Julian Assange
OpEd News summarizes highlights