Via Calculated Risk, a report that the FBI knew about mortgage fraud but couldn't do anything because they were too busy with counter-terrorism:
"It is clear that we had good intelligence on the mortgage-fraud schemes, the corrupt attorneys, the corrupt appraisers, the insider schemes," said a recently retired, high FBI official. Another retired top FBI official confirmed that such intelligence went back to 2002.
The problem, according to the two FBI retirees and several other current and former bureau colleagues, is that the bureau was stretched so thin that no one noticed when those lenders began packaging bad mortgages into bad securities.
... Both retired FBI officials asserted that the Bush administration was thoroughly briefed on the mortgage fraud crisis and its potential to cascade out of control with devastating financial consequences, but made the decision not to give back to the FBI the agents it needed to address the problem. After the terrorist attacks of 2001, about 2,400 agents were reassigned to counterterrorism duties.
Keep in mind Osama bin Laden's goal: hurt America. What does al-Queda care if they hurt us directly or if they get us to hurt ourselves? Probably not much at all. If they distract the police so much that crime generally rises, the criminals win.
On the other hand, given that so much of the financial disaster in this country has benefitted the super-rich people the previous administration coddled, perhaps they weren't simply incompetent.