Investigators say the 17th Street Canal levee was built on very soft peat. They believe the Army Corps of Engineers did not drive the sheet piling down deep enough to anchor it. The Corps disputes this. (Army Corps records show the sheet piling was designed to go 10 feet down, but recently the Corps has said that it reaches a depth of 17 feet.)

Investigators think that when Hurricane Katrina hit, the storm surge pushed through the peat beneath the levee and knocked it off its foundation.
Image: Doug Beach for NPR