According to Associated Press:
BREAKING: Official tells AP: Trayvon Martin's killer to be charged with 2nd-degree murder; is in custody. -RAS
— @AP via SocialFlow
NBC also reported Zimmerman will be charged and is in custody. The special prosecutor will hold a 6 PM ET news conference. We'll post video and live updates then.
2:43 PM PT: AP says its source is a "law enforcement official." Their report is here.
2:44 PM PT: CNN also says Zimmerman is in custody, though they aren't reporting the second-degree murder charge that AP says will be filed.
2:51 PM PT: More on second-degree murder in Florida:
Under Florida law, second degree murder is the unlawful killing of a person when perpetrated by any act imminently dangerous to another and evincing a depraved mind, regardless of human life, although without any premeditated design to effect the death of any particular individual. The maximum sentence for the crime is life in prison.