The Bush administration has protected its ranks from many investigations that could have shown illegal actions; they have redacted or outright destroyed many potentially incriminating documents. Some of the "shredding" or other document destruction was not legal; some of it has been, for the moment, protected by the overextension of so-called "executive privilege" (by the same factions who wanted the Secret Service to be subpoenaed to testify against Bill Clinton in the Lewinsky matter).   
I'd add that the Democratic leadership in both the House and Senate have been way too reticent to undertake Congressional investigation of suspected illegal actions of people in the G B Bush administration.   The firing of attorneys in the Federal Dept. Of Justice on political grounds and replacing them with highly partisan attorneys is just one of the areas where, even if this were to be fully adjudicated and found to be legal, it's still political horse-trading and putting partisan goals above the even-handed execution of law.