The report found that while the department's review system for classifying documents needed improvement, no federal laws had been broken in the incident.
In response to a Freedom of Information Act request filed by The Associated Press and other news media, the Treasury Department's inspector general released several hundred pages covering its investigation of how O'Neill received some 19,000 documents which were used to write a book highly critical of President Bush.
The new report found that 140 of those documents had not been marked classified even though they contained national security or sensitive but unclassified information.
"Had these 140 documents been properly marked as classified, the documents would not have been entered into Treasury's unclassified computer system and O'Neill would not have received them," the report said.
Treasury launched an investigation into the documents in January after CBS's "60 Minutes" showed a document marked "secret" during an interview in which O'Neill promoted the new book, "The Price of Loyalty." [Simon and Schuster, the book's publisher, and CBSNews.com, are both units of Viacom.]
O'Neill turned over the 19,000 documents he was provided to former Wall Street Journal reporter Ron Suskind for the book, which painted an unflattering portrait of Mr. Bush and the White House decision-making process based on O'Neill's two years in the administration.
O'Neill was fired on Dec. 6, 2002, after Mr. Bush decided to shake up his economic team.