"He's the defending champion, so everyone will be looking for him to come back and do it again. But we have a lot of sprinters coming up to challenge him," Powell said. "So it's not going to be as easy as Beijing.
"Definitely, he has to watch his back."
AP National Writer Eddie Pells in Kingston, Jamaica, and AP Sports Writers Rob Harris in London and Beth Harris in Omaha, Neb., contributed to this report.
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