The evidence is suggestive, but not totally conclusive. The city of San Antonio has not tried very hard to learn whether or not victims and perpetrators came from New Orleans. Houston did, and got federal money. Both cities saw a spike in crime after the refugees arrived. San Antonio's crime rate had been falling prior to their arrival. Here are the articles:
Katrina crime: Perceived or real?
S.A. killings show few ties to storm.
Remember, to prove cause and effect, you need ceteris paribus conditions. Does this analysis meet that criteria?
UPDATES. Ken Rodriguez, a columnist for the Express-News, thinks the answer is yes. Here is the link
Ken Rodriguez: Here's the unvarnished truth about Katrina crime: It happened
Here are two more articles from the Express-News
In one area with evacuees, robbery is routine
Complex with Katrina evacuees became a criminal haven