Medical Examiner Contradicts A Key Part Of George Zimmerman's Story

A medical examiner's testimony that George Zimmerman's head injuries were "minor" could undermine his claims that he shot an unarmed 17-year-old because he feared for his life.

Jacksonville, Fla. medical examiner Valerie Rao testified Tuesday that it was unlikely Trayvon Martin "slammed" George Zimmerman's head into the ground as Zimmerman claims he did before he shot the teen in February 2012.

"The injuries are so minor," Rao testified, "and to me, slamming implies great force."

Rao also testified that the cuts on Zimmerman's head are consistent with "one impact" and that the injuries were not life-threatening.

Zimmerman, who's charged with second-degree murder, says he followed Martin because he seemed suspicious. After Zimmerman approached Martin, the teen decked him and pounded his head on the sidewalk, according to Zimmerman's account.

Upon tense cross-examination from Zimmerman's lawyer Mark O'Mara, Rao remained fairly confident Zimmerman's head hadn't been slammed repeatedly — though she did say it was "possible" his head had been hit more than once.