What was the U.S. hiding?

I remember back in the run-up to the invasion of Iraq, Saddam's government put out a required report to the U.N. detailing their weapons programs. Before the report could reach the U.N., American officials swooped in and grabbed it. A few days later the report was returned, with large portions (about 1/3) redacted or removed. Has there ever been any clue to what was censored from that report? Also, why was this obviously illegal act never addressed?