The NAACP is asking for federal involvement:
In a letter sent to U.S. Attorney General Eric Holder, the NAACP expressed doubt in the Sanford Police Department and asked the Department of Justice to review the case."The NAACP has no confidence that, absent federal oversight, the Sanford Police Department will devote the necessary degree of care to its investigation. We therefore call upon you to detail personnel to Sanford immediately to review the facts, ensure that the Sanford Police Department conducts an impartial, thorough and prompt investigation of the circumstances involving the death of this unarmed teen, and ensure that the responsible person is held accountable if a crime was committed," the letter said.
For more on the legalities of all this I recommend this thread from yesterday. My sense is that the law in Florida really opens the door for vigilantism. It doesn't do much good to cite law from around the country. Florida is rather particular.
Also more on Zimmerman's arrest from 2005:
Zimmerman has a single arrest. When he was a 21 and a UCF student, one of his friends was being arrested by state alcohol agents for serving underage drinkers, according to his arrest report. Zimmerman began talking to the suspect, and when agents ordered him to stop and tried to escort him away, he became profane and pushed the agent's hands, the report said."After a short struggle," the report said, Zimmerman was handcuffed and arrested. Prosecutors charged him with resisting arrest without violence, and he entered a pretrial-diversion program, something common for first-time nonviolent offenders, who then wind up with no conviction on their records.