UPDATE: 1:40 p.m. ET: Trayvon Martin shooter George Zimmerman appeared in a Florida courtroom shortly after 1:30 p.m. He entered the courtroom in handcuffs.

Flanked by a police officer and his lawyer, Mark O'Mara, Zimmerman acknowledged the second-degree murder charge against him. He spoke only briefly, saying "Yes, sir" twice, and did not enter a plea.

The judge agreed there was probable cause to proceed, and set for a formal arraignment on May 29, at 1:30 p.m. ET. O'Mara said Zimmerman would plead not guilty.

Zimmerman is being held in protective custody without bail in Seminole County Jail. O'Mara did not ask the judge to set a bond, but asked for the case file to be sealed. The prosecutors agreed to the request.

1:20 p.m. ET: Trayvon Martin's mother Sybrina Fulton, who said on the "Today" show early Thursday that she believed Zimmerman's fatal shooting of her son was an "accident," has retracted those comments.

"George Zimmerman stalked my son and murdered him in cold blood," told MSNBC in a separate interview.

"I believe it was an accident," Fulton told Ann Curry in the initial interview. "I believe that it just got out of control and he couldn't turn the clock back."

1:10 p.m. ET: George Zimmerman is due in court at 1:30 p.m ET. Watch the video below.

12:43 p.m. ET: Is George Zimmerman safer in jail or out? That's the question the judge overseeing the second-degree murder charge against him will consider at his arraignment later today, when Zimmerman's attorney, Mark O'Mara, plans to ask the judge to release his client on bond.

"You could make an argument that he would be safer to remain in custody," Charles Rose, director of the center for excellence and advocacy at the Stetson University College of Law in Gulfport, Fla.,told CNN.

"I think nobody would deny the fact if George Zimmerman is walking down the street today, he would be at risk," O'Mara, said Wednesday.

But the judge may opt to release Zimmerman because of the "nightmare" it would be to guarantee his safety in jail, Rose said.

And while lawyers for Trayvon Martin's family contend Zimmerman is a "flight risk," Rose said that given the intense media coverage of the case, Zimmerman's face is too recognizable for him to be able to flee.

"I would not be surprised to see him walk in the door and walk out the door," Rose said.

12:20 p.m. ET: According to the Orlando Sentinel, George Zimmerman "can ask to have the second-degree-murder charge against him dropped without having to stand trial in the death of Trayvon Martin."

The paper reports that in 2010, the Florida Supreme Court "ruled that anyone claiming 'stand your ground' immunity in a death, battery or assault case can request a hearing on the evidence."

However, there is no indication Zimmerman will do so. His attorney, Mark O'Mara, said Zimmerman will plead "not guilty" at his arraignment later today.

11:00 a.m. ET: George Zimmerman's attorney Mark O'Mara said he's concerned about his client's safety, but would like the judge to grant bail because he needs Zimmerman's assistance in building a case outside of jail.

"I would like to get him out," O'Mara said in an interview with Soledad O'Brien on CNN. "I need him out to assist me in going over all the evidence and preparing our defense. I'm concerned about his safety to a certain extent, but I'm truly hoping that there will be a receding of the frustrations or anger now that the process is moving forward. ... There are certainly still some animosities and some high emotions around the case."

O'Mara also dismissed the assertion by Zimmerman's former legal team that his client is suffering from post-traumatic stress disorder: "A diagnosis like that should be made by a professional psychologist or psychiatrist.  I'm not going to make a diagnosis like that. He is stressed. He's tired. He's been through a lot with the way this case has been handled. I'm just hoping that his mental health stays well and that we can move forward with getting the case figured out."

George Zimmerman's booking photo provided by the Sanford Police Department. (AP)

8:36 a.m. ET: George Zimmerman is expected to be arraigned Thursday on the second-degree murder charge in the shooting death of Trayvon Martin.

According to his attorney, Mark O'Mara, Zimmerman will plead not guilty. O'Mara said he will ask the judge to set a bond for his client, who is being held in protective custody without bail in Seminole County Jail.

Martin's parents, Tracy Martin and Sybrina Fulton, spoke out early Thursday saying they just want justice for their son.

"I believe it was an accident," Fulton said on the "Today" show. "I believe that it just got out of control, and he [Zimmerman] couldn't turn the clock back."

"We just want him [Zimmerman] to be held accountable for what he's done," Fulton said. "And by him not being arrested, that would not have been done. So we are happy that he was arrested so that the committee gives his side of the story."

Zimmerman, the 28-year-old neighborhood watchman who shot and killed Martin on Feb. 26 in Sanford, Fla., told police he was acting in self-defense, and that Martin attacked him.

The criminal charge was announced on Wednesday by Angela Corey, the special prosecutor in the case.

"Three weeks ago our prosecution team promised [Martin's] sweet parents we would get answers to all of their questions, no matter where our quest for the truth led us," Corey said at a news conference in Jacksonville. "And it is that search for justice for Trayvon that has brought us to this night." LINK