Seminole County Sheriff Don Eslinger had one message for fellow Floridians about the George Zimmerman trial verdict: Destruction and violence after a verdict will “not be tolerated.”

It was a preemptive warning ahead of what some some authorities worry may be a tinderbox situation. A six-woman jury began Friday to deliberate the case of Mr. Zimmerman, a neighborhood watch captain charged with the murder of unarmed black teenager Trayvon Martin.

Late Friday, according to the Orlando Sentinel, “sign-wavers and others have gathered peacefully near a fountain outside the courthouse [in Sanford, Fla.] in an area designated for people to express themselves.”

Social media armies on both sides of the case are gearing up for the verdict, media coverage is nonstop, and emotions are running high in many quarters. Even so, this moment in America lacks many of the propellants that stoked racially charged riots in the past, such as the Los Angeles riots in 1992 and the Oakland, Calif., riots of 2009, says a historian who studies racial unrest.

“History shows that there need to be large underlying socioeconomic conditions that will serve as the tinder for the spark that will cause the eruption,” says Michael Flamm, a professor of political history at Ohio Wesleyan University.

“There is, of course, plenty of economic distress and plenty of social distress in Sanford and in Florida and in the country as a whole, but I’m not sure it’s at a critical mass right now,” he adds. “There’s also a sense in most communities that a riot or rebellion in response to a miscarriage of justice is both ineffective and counterproductive – after all, it’s your community that’s going to be most adversely affected. Police are not going to allow you to go and riot or loot in other communities.”

Police not just in Sanford, but also in Miami and other Florida cities have circulated memos and made preparations – including putting up “First Amendment zones” – amid concern that a not-guilty verdict could ignite unruly protests or even street violence.

In Miami, authorities have released a pre-verdict video that urges people to raise their voices, not their fists, and the city plans to set up several First Amendment zones where people who want to vent can do so in rallies and debate.

“Riots are not acceptable, and riots are not expected,” Miami-Dade Police Director J.D. Patterson said from the pulpit at Peaceful Zion Missionary Baptist Church on Thursday night, reported Miami’s CBS-4 TV.

Such statements nevertheless underscore a cause for concern, especially because the nuances of the actual trial – and the jury’s task – may be lost to the public at large amid outrage and rumors, says Mr. Flamm of Ohio Wesleyan University.

Although a jury must decide whether the state proved “beyond a reasonable doubt” that Zimmerman committed murder or manslaughter, some might interpret a not-guilty verdict this way: A jury of five whites and one minority agreed that Trayvon Martin, who was walking by himself, unarmed, “minding his own business,” as prosecutors said, was responsible for his own death, and that George Zimmerman, his killer, is “factually innocent” and should go free.

In the aforementioned riots in Los Angeles and Oakland, racial violence sprang up around concerns of police brutality. A new documentary about the Oakland riots, "Fruitvale Station," depicts the aftermath of the death of Oscar Grant, shot in the back by a transit cop at a subway station. Shocked train riders recorded the incident on their phones and promptly posted the video and images online, where they went viral.

The appearance of "Fruitvale Station" at the culmination of the Zimmerman trial rings a powerful theme. “The deaths of these and other African-American young men (Mr. Grant was 22) touch some of the rawest nerves in the body politic …,” writes A.O. Scott, a film critic at The New York Times.

But important differences exist between Oakland and Sanford. The Oakland BART riots began with an apparent act of police brutality – shooting a man in the back as he was lying down. In the Trayvon Martin case, the concern, rather, is that police were indifferent to the death of a black teenager. The decision by Sanford police not to charge Zimmerman triggered widespread protests, and 44 days later a state prosecutor indicted Zimmerman on murder charges. Since then, the trial has served as more of a civics lesson than as a call to arms, some legal experts suggest.

“A lot of it depends on your perspective: Is the system actually trying to bring about justice, or is it trying to sweep it under the rug?” says Flamm. “[The Trayvon Martin case] is not a clear case where the system is trying to sweep what happened under the rug.”

Nevertheless, Florida has had its share of race riots, both near Sanford and in Miami.

For the time being, police say they’re preparing for civil unrest, although they do not sense it is imminent. “While this case has brought a great deal of emotion, there is no tension in Seminole County,” Sheriff Eslinger said on Friday afternoon.

Nevertheless, Eslinger added, “we encourage businesses to not disrupt operations, we encourage all residents to live their lives normally, and that we will not tolerate anyone who uses this verdict as an excuse to violate the law.”

Sanford Police Chief Cecil Smith, who took over after Chief Bill Lee was fired over the handling of the Zimmerman case, said, “I’m not going to say that everything is peachy-keen in Sanford.” But he noted that the Trayvon Martin case provides “a great opportunity for evolution for Sanford, [where we] make sure we have the opportunity to speak our piece peacefully. There’s nothing on the horizon for us other than move forward.”

Flamm is interested to see how the verdict will ripple through social media. Already, thousands of Facebook users are blacking out their profile pictures in solidarity with Trayvon’s family. The trial has been buzzing on Twitter and other social networks for weeks.

“If Zimmerman walks, the day after the verdict we are calling for a day of absence, a total national blackout,” write the organizers of one online protest, which had received tens of thousands of pageviews. “Call in sick and tired. Call in angry, call in crazy. But call in.”

Civil rights leader Jesse Jackson, who helped propel the “Justice for Trayvon” and “Million Hoodie Marches” last year, also took to Twitter, where the hashtags #Trayvon and #Zimmerman were among the most popular of the moment.

"If #Zimmerman is not convicted, avoid violence because it only leads to more tragedies. Self-destruction is not the road to reconstruction," the Rev.Mr. Jackson tweeted.

Such missives are a counterpoint to the Oscar Grant shooting in Oakland, where online videos of the shooting went viral, fueling outrage that morphed into street violence. When the verdict in the Zimmerman case comes, it's possible that “social media … could curtail speculation and rumors that lead” to riots, says Flamm.