What moved Trayvon Martin's Feb. 26 death from a local story to a national tale that has sparked a discussion about racial profiling and race relations?

Social media played a critical role. And there were key moments along the way.

First. NBC News today reports on Kevin Cunningham of Washington, D.C., who it says started the online petition at Change.org demanding that authorities arrest George Zimmerman. He's the 28-year-old Sanford, Fla., neighborhood watch volunteer who killed 17-year-old Martin in what Zimmerman says was a case of self defense — but Martin's families and supporters feel was racial profiling. Martin was black.

Cunningham, who is white, had read about Martin's death on a listserv for "Men of Howard." That's "an informal, secretive fraternity that he joined while attending the historically black Howard University as a law student," NBC says. On March 8, NBC says, Cunningham went to Change.org to create the online appeal.

Second. After a few days, the petition had 10,000 "signatures." It was then, Cunningham told NBC, that Change.org asked if it could be transferred to the control of Martin's parents. Cunningham said yes.

The parents' emotional appeal seemed to supercharge the petition effort. "On February 26, our son Trayvon Martin was shot and killed as he walked to a family member's home from a convenience store where he had just bought some candy," it begins. "He was only 17 years-old."

"Brianna Bayo-Cotter, Change.org's communication's director, says this is their largest online petition drive ever," Time's NewsFeed blog reports.

As of this afternoon, there are about 2.2 million digital signatures.

Third. Meanwhile, "a host of celebrities from hip hop icon and entrepreneur Russell Simmons to new age leader Deepak Chopra to director Spike Lee" started posting messages on Twitter, Facebook and other sites about the case, as ABC News recounts. And much as with the recent viral spread of the "Kony 2012" campaign about an African warlord, those messages resonated. (Related note: Lee retweeted what he thought was Zimmerman's address. He was wrong and is now apologizing to the elderly couple who live at the address he posted, who received threats and hate mail. They might have cause to sue him.)

Overall, as NewsFeed says, "social-media watchers call the viral growth regarding Trayvon Martin the 'perfect storm.' With a frustrated population that perceives an injustice and an easy platform for expression, it has all the elements needed for a widespread outcome."

Other factors obviously played roles. Parents, especially those with African-American sons, identified with the loss suffered by Martin's mother and father. The release of 911 tapes from Zimmerman's calls to police gave the news media dramatic audio that could be played over and over again. Cable news networks saw a story of high interest to viewers that they could devote lots of time to. "The hoodie" is an image that can be easily short-handed.

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