A key twist in the story, said Julison, was the release on March 16 of tapes of Zimmerman’s 911 emergency calls. The tapes, which Sanford police had resisted releasing, gave news outlets fresh material to report, and added another emotional element to the story. One recording captured screams for help in the background. “It humanized the situation,” he said. “You hear people crying. You can’t help but be moved by it.”

By this time, the story had spread to social media, with such celebrities as Spike Lee, Russell Simmons and Mia Farrow tweeting their outrage, and LeBron James and his Miami Heat teammates posing for photos in hoodies — the garment worn by Martin at the time of his death.

The Martin family, in New York for an appearance on “The Today Show,” also agreed to participate in a local rally dubbed “The Million Hoodie March,” which drew enormous media attention. President Obama finally seemed to certify the story’s national significance March 23 when he commented, “If I had a son, he’d look like Trayvon.”