Findings based on physical evidence, Army Corps of Engineers documents and hydrodynamic models run through a Louisiana State University supercomputer show two of three levees collapsed when weakened soils beneath them became saturated and began to slide.
The findings also confirmed that a little-used navigation canal helped amplify and intensify Katrina's initial surge on Aug. 29, contributing to a third floodwall collapse on the east side of town.
The Washington Post said experts now believe Katrina was no stronger than a Category 3 storm when it came ashore.
"This was not the Big One -- not even close," said Hassan Mashriqui, a storm surge expert at LSU's Hurricane Center. He said that Katrina would have caused some modest flooding and wind damage regardless, but that human errors turned "a problem into a catastrophe."
The floodwall failures left about 100,000 homes underwater and caused most of Louisiana's approximately 1,000 hurricane deaths.