This Alabama state senator has the answer:

Hurricane Katrina and other storms that battered the Gulf Coast were God’s judgment of sin, according to state Sen. Hank Erwin, R-Montevallo.
“New Orleans and the Mississippi Gulf Coast have always been known for gambling, sin and wickedness,” Erwin wrote this week in a column he distributes to news outlets. “It is the kind of behavior that ultimately brings the judgment of God.”
After touring Gulfport and Biloxi, Miss., and Bayou La Batre, Erwin said he was awed and humbled by the power of the storm. But he wasn’t surprised.
“Warnings year after year by godly evangelists and preachers went unheeded. So why were we surprised when finally the hand of judgment fell?” Erwin wrote. “Sadly, innocents suffered along with the guilty. Sin always brings suffering to good people as well as the bad.”

In case you doubt erwin’s credentials there is this:

Twinkle Andress, executive director of the Alabama Republican Party, said she had not seen Erwin’s column. But she praised his performance as a senator.
“Obviously, I think Hank Erwin is a great senator and been a real leader,” she said.

According to the article erwin is in fine company: al-Qaida, Pat Robertson and Louis Farrakhan have expressed similar views.