From the Chicago Tribune:
New information compiled by hurricane researchers suggests the system struck southeast Louisiana on Aug. 29 with peak-sustained winds of 115 miles per hour. That would have made it a Category 3 storm, still a major hurricane but a step down from the enormous destructive force of a Category 4.
Katrina might have further downgraded to a strong Category 1 system, with 95 m.p.h. winds, when it punched water through New Orleans' levees, severely flooding most of the city and killing nearly 1,000 people. The levees were designed to withstand a Category 3 storm.
If verified, the information, compiled by the National Oceanic and Atmospheric Administration's Hurricane Research Division, could have chilling ramifications.
Notably, it would mean that if a Category 4 or a Category 5 hurricane were to hit the same region it would be even more catastrophic.