Law enforcement officials in Sanford, Fla., investigating last month's shooting of an unarmed black teen by self-appointed neighborhood watch leader George Zimmerman say they may have missed a possible racist remark made by Zimmerman to a 911 operator seconds before he shot 18-year-old Trayvon Martin.

The officials made the admission to ABC News after revealing that Zimmerman can be heard on a recording of one of the 911 calls made on the night of the shooting saying what some people think sounds like "f***ing coons" under his breath.

Moments later Zimmerman confronted Martin and shot him dead after a brief scuffle, , according to investigators.

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WFTV had an audio expert listen to the call, and determined that the word said was "punks."

Regardless of whether Zimmerman is saying "punks" or "coons," MSNBC host Lawrence O'Donnell believes the remarks do suggest a cover-up on the part of the Sanford Police Dept.

"When I heard that tape today and when I interpreted it the way attorney [Jasmine] Rand interpreted it, I believe that what we have here is evidence of a police cover-up," O'Donnell said. "This is not conclusive legal proof of a police cover-up, but this is evidence that the police, that local police department, never wanted anyone to hear those two words. And that's why we haven't heard those two words until today."

In the moments before Martin was shot, he was on his cellphone talking to his girlfriend.

"He said this man was watching him, so he put his hoodie on. He said he lost the man," the girl told ABC News. "I asked Trayvon to run, and he said he was going to walk fast. I told him to run but he said he was not going to run."

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