The U.S. attorney must consider the "nature and seriousness of the offense" in deciding whether to prosecute or not. The major factor that determines that is "the actual or potential impact of the offense on the community and on the victim."
Martin's family, their attorneys, and civil rights leaders face the terrible reality that if Zimmerman walks, there will be little recourse from the feds. It's not just. It's not fair. But, unfortunately, it's the system.
If you hear some uninformed person say the government should stop going after suspect athletes tell them to check their hero-worship at the door. In America, everyone suspected of a crime is supposed to be investigated equally.