TALLAHASSEE, Fla. (AP) -- A law enforcement official says that the shooter in the Trayvon Martin case will be charged with second-degree murder and is in custody.

The official with knowledge of the case says that the charge against 28-year-old George Zimmerman will be announced at a news conference at 6 p.m. Wednesday. The official says he's in custody in Florida but wouldn't say where. The official spoke on condition of anonymity because he wasn't authorized to disclose the information.

An arrest had been delayed because of Florida's so-called stand your ground law, which gives people wide latitude to claim self-defense in a killing and other altercations.

The lack of an arrest in the 17-year-old's death had sparked outrage and rallies for justice in the Orlando suburb and across the country.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

After weeks of mounting tension and protests across the U.S., a special prosecutor has decided to bring charges against neighborhood watch volunteer George Zimmerman in the killing of 17-year-old Trayvon Martin, a law enforcement official said Wednesday.

The official, speaking to The Associated Press on condition of anonymity, said prosecutor Angela Corey would announce the charges at a 6 p.m. news conference. The official said he did not know the charges. Corey confirmed only that she would make an announcement at that time.

The Martin family and their attorney, Ben Crump, renewed their pleas for calm as the hour approached.

"We don't need anybody taking these matters into their own hands," Crump said.

The law enforcement official said that authorities knew where Zimmerman was and were planning to arrest him soon rather than let him turn himself in. The official said he was not in Florida.

Zimmerman's shooting of the unarmed black teenager Feb. 26 brought demands from black leaders for his arrest and set off a furious nationwide debate over race and self-defense that reached all the way to the White House.

The 28-year-old Zimmerman, whose father is white and whose mother is Hispanic, said the teenager attacked him. Martin's family argued Zimmerman was the aggressor.

On Tuesday, Zimmerman's lawyers announced they were withdrawing from the case because they hadn't heard from him since Sunday and didn't know where he was. They portrayed his mental state as fragile.

"He is largely alone. You might even say he is emotionally crippled by virtue of the pressure of this case," said one of the lawyers, Hal Uhrig.

The case has drawn the interest of the highest levels of the Obama administration, with the Justice Department's civil rights division opening its own investigation.

Zimmerman's arrest was delayed partly because of Florida's "stand your ground" law, which gives people wide leeway to use deadly force without having to retreat in the face of danger. Florida is among 21 states with such a law.

Tensions have risen in recent days in Sanford. Someone shot up an unoccupied police car Tuesday as it sat outside the neighborhood where Martin was killed. And a demonstration by college students closed the town's police station Monday.

Six weeks ago, Martin was returning to the home of his father's fiancee from a convenience store when Zimmerman started following him. Zimmerman told police dispatchers he looked suspicious. At some point, the two got into a fight and Zimmerman used his gun.

Zimmerman told police Martin attacked him after he had given up chasing the teenager and was returning to his truck. He told detectives that Martin knocked him to the ground and began slamming his head on the sidewalk. Zimmerman's father said that Martin threatened to kill his son and that Zimmerman suffered a broken nose.

A video taken about 40 minutes after the shooting as Zimmerman arrived at the Sanford police station showed him walking unassisted without difficulty. There were no plainly visible bandages or blood on his clothing, but Zimmerman may have had a small wound on the back of his head.

The shooting ignited resentment toward the police department, and Police Chief Bill Lee temporarily stepped down to let passions cool.

Civil rights groups and others have held rallies around the country, saying the shooting was unjustified. Many of the protesters wore the same type of hooded sweatshirt that Martin had on that day, suggesting his appearance and race had something to do with his killing.

President Barack Obama injected himself into the debate, urging Americans to "do some soul-searching." "If I had a son, he'd look like Trayvon," Obama said March 23.

The local prosecutor disqualified himself from the case, and Gov. Rick Scott appointed Corey, the prosecutor for Jacksonville, to take it over.

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Farrington reported from Tallahassee, Fla.