http://www.latimes.com/news/nation/...rman-hoa-newsletters-20120518,0,2949112.story
"The documents also show that a Sanford police investigator, Trekell Perkins, received a phone call from an anonymous woman two days after the shooting who said that Zimmerman had "racist ideologies," and was "fully capable" of instigating a deadly confrontation."
Few knew much about this case on February 28th, 2012. It had to be someone privy to the fact that GZ shot TM and had already been sent home to freely go about his careless life. IMO, of course.
It's listed as the very next thing that happened after Tracy Martin was at the police station concerned that George hadn't been arrested yet. So maybe it was someone much closer to the event -- like a disgruntled friend or family member of the Martins.