What was the main cause of the severe casualties?

W: Welcome to the 9 o’clock news of CDB, I’m Susan Morphy. A tornado hit the northwestern parts of Mexico, and caused severe casualties. Though there had been warnings beforehand, as the tornado went slightly away from the predicted route, many people failed to escape from its fatal sweeps. According to the first estimate, at least 200 people were killed and about 30,000 became homeless. Let’s now connect to Tim Hudson, our correspondent there, for some more details. Hi, Tim. Can you give us a report about the tornado there?
M: Hi, Susan. As you all know, the tornado has caused great losses. There had been some warnings about the tornado before it arrived. But it came slightly away from the predicted route, and, moreover, it came to the densely populated area about 2 AM, when most people were in sound sleep. A lot of buildings have become topless, and many large trees have been rooted up.
W: What about the casualties?
M: There has been no exact report about it so far. According to some reliable estimate, there were no less than 200 of them. As a heavy rain went with tornado, some parts of the city where I’m standing have been cut off and isolated by the flood. We don’t know what had exactly happened to those parts.
W: How about the rescue work?
M: As a matter of fact, the local government wasted no time. Just about 20 minutes after the tornado had passed, the local TV and broadcast started to give instructions on the locations of emergency centers in different districts of the city. Early this morning, a team from the Mexican government has arrived. Actually, I’m just back from the airport.
W: Thank you, Tim. Now let’s move on to other news...