What seemed like a single shooting incident in a midsize Florida city — the killing in February of Trayvon Martin — now threatens to divide a country. How did that happen?


How did a discrete local event take on the scale of a presidential election, or assassination, with every shred of information subjected to Zapruder-like deconstruction? The photo, the hoodie, the tweets, the Skittles — all have taken on outsize importance in a supercharged national debate, while perspective and restraint get left by the wayside. An audience weaned on decades of police procedurals on television has become a crowd-sourced judge and jury.


Somehow, Mr. Martin’s death has become a meme, like Obamacare, and Etch A Sketch, and the now-familiar partisan free-for-all in the media has ensued.


That the public is rendering its verdict immediately and firmly may be routine, but choosing sides takes on a deeper, more dangerous meaning when race is at the heart of the story. Race as an explosive issue is nothing new, but it’s been staggering to see it simmer and boil over in our hyperdivided media environment where nonstop coverage on the Web and cable television creates a rush to judgment every day.


Partisan politics and far-flung conflicts fit nicely into that world — who’s ahead, who’s behind, should we stay or go? — but racial conflict? Not so much.


That hasn’t stopped many in the media from displaying the same reflexive vigilantism that some are attributing to George Zimmerman, the man who shot Trayvon. All over the Internet and on cable TV, posses are forming, positions are hardening and misinformation is flourishing. Instead of debating how we as a culture are going to proceed, an increasingly partisan system of news and social media has factionalized and curdled.


“It has been depressing to watch something as important as this get run through the American polarization machine,” said Chris Hayes, the host of “Up With Chris Hayes,” a weekend political talk show on MSNBC. “The first week after it became national news, Act 1, seemed to be built on a shared agreement that what happened was outrageous and upsetting no matter what the facts ended up showing. But then came the backlash and now you’ve got people picking sides.”


Yes, there have been significant efforts at playing it up the middle: People magazine put it on the cover and called it “An American Tragedy,” as if to remind everyone that we all lose something when a child dies. And ABC did not play “gotcha” with the police security video it had acquired of Mr. Zimmerman.


But if we have learned anything in the last few years, it is that traditional media are now only in charge of part of the story. There is a paucity of facts and an excess of processing power because everyone with a keyboard is theoretically a creator and distributor of content. Most of those efforts begin from behind a firmly established battle line, then row backward to find the facts that they need. Was that a dark spot on the back of George Zimmerman’s head in the grainy police video, or evidence of a beat-down? We retweet and “like” what we agree with and dismiss the rest.


As if the overheated cable news debate weren’t enough, social media are fueling the story with misinformation, along with incendiary calls to action. There is a Twitter account called “@killzimmerman” that suggested George Zimmerman needed to be “shot dead in the street.” On Twitter, the movie director Spike Lee passed on what he thought was Mr. Zimmerman’s address, but it was wrong and an elderly couple was forced to flee from their home. And what if Mr. Lee had gotten it right? (Mr. Lee has since apologized and reached a settlement with the couple.)


Early last week, thanks to Fox News and Geraldo Rivera, coverage pivoted around the preponderance of hoodies rather than the ubiquity of handguns. By the end of last week, the Drudge Report was in lurid, link-driven dudgeon, suggesting that the real victim was George Zimmerman. On Thursday afternoon, there were more than 10 links at the top of the site to articles casting doubt on just how much of a victim Trayvon was, including an interview with Mr. Zimmerman’s father accusing President Obama of spreading hate. It’s ugly out there and getting uglier.