To blame Ashcroft for 9/11 is ridiculous. No one could have forseen the events that took place. Yes there were signs but putting the dots together with the lack of information sharing between agencies made it almost impossible. That is what Ashcroft accomplished taking the lessons we learned and applying them to today's needs in regards to law enforcement.
See DOD memo: Wolfowitz to Runmsfeld 9/15/04 "Were We Asleep?"