Many attorneys said they had expected the prosecutor to opt for the lesser charge of manslaughter. The most severe homicide charge, first-degree murder, is subject to the death penalty in Florida and requires premeditation - something all sides agreed was not present in this case.
"I predicted manslaughter, so I'm a little surprised," said Michael Seigel, a former federal prosecutor who now teaches law at the University of Florida. "But she has more facts than I do."
O'Mara, Zimmerman's attorney, said his client would plead not guilty and invoke Florida's so-called "stand your ground" law, which gives people wide latitude to use deadly force rather than retreat during a fight.
The confrontation took place in a gated community where Martin was staying with his father and his father's fiancée. Martin was walking back in the rain from a convenience store when Zimmerman spotted him and called 911. He followed the teenager despite being told not to by a police dispatcher and the two got into a struggle.
Zimmerman told police Martin punched him in the nose, knocking him down, and then began banging the volunteer's head on the sidewalk. Zimmerman said he shot Martin in fear for his life. Sanford police took Zimmerman, whose father is white and whose mother is Hispanic, into custody the night of the shooting but released him without charging him.
A judge could dismiss the charge based on the "stand your ground" law, legal experts said. But some experts say the judge will also be under tremendous pressure to let the case go forward.
"Judges are not likely to take that out of the hands of the jury," said Florida defense attorney Randy Reep.
Other attorneys weren't surprised that Corey went for the maximum.
"Prosecutors look for leverage. They'll typically overcharge knowing that gives them wiggle room for plea discussions," said Derek Byrd, incoming president of the Florida Association of Criminal Defense Lawyers. "She knows that she could offer him manslaughter at some point or get in front of a jury that could split the verdict and agree on a lesser offense."
Corey wouldn't discuss how she arrived at the charges or disclose other details of her investigation. The prosecutor in Jacksonville was appointed to handle the case by Republican Gov. Rick Scott after the local prosecutor disqualified himself.
The U.S. Justice Department's civil rights division is conducting its own civil rights investigation.
Tensions had risen in Sanford, a town of 50,000 outside Orlando. Someone shot up an unoccupied police car Tuesday outside the neighborhood where Martin was killed. Outside of Sanford City Hall on Wednesday, Stacy Davis, who is black, said the arrest "is not a black or white thing for me. It's a right or wrong thing."
In Washington, Martin's family pleaded for calm in response to the decision. But Martin's mother, Sybrina Fulton, clasped hands and smiled in relief when she heard Corey utter the words "second-degree murder" on television.
"We wanted an arrest and we got it," Fulton said later. "Thank you Lord, Thank you, Jesus."