TRAYVON MARTIN, a 17-year-old African American, has been dead for nearly one month, killed Feb. 26 in Sanford, Fla., by George Zimmerman. Mr. Zimmerman is a heat-packing neighborhood watch captain who frequently called police to report suspicious sightings of black boys in the gated community where he lived and patrolled.

The state prosecutor in the Central Florida county that includes Sanford announced Tuesday that a grand jury would convene next month — six weeks after the shooting. This grudging decision came only after Mr. Martin’s family members began to speak publicly about the disturbing circumstances of his death; only after released 911 recordings showed Mr. Zimmerman stalking Mr. Martin as the youth walked home with an iced tea and a bag of Skittles, purchased from a nearby convenience store; and only after the Justice Department declared that it would step in to determine whether federal civil rights laws were broken.

Even now, with much of the country watching this sleepy outpost one hour north of Walt Disney World, there is reason to worry whether justice will be served. Florida and some 16 other states have so-called “Stand Your Ground” laws that allow individuals to use deadly force if they have a “reasonable” fear of grave harm or death from a perceived attacker. Mr. Zimmerman claimed he shot in self-defense when attacked by Mr. Martin. A purported eyewitness — presumably a neighbor of Mr. Zimmerman’s — told a local television station that he saw two men fighting near his home and suggested that Mr. Martin was hitting Mr. Zimmerman. This account contrasts with that of Mr. Martin’s girlfriend, who was speaking with him by cellphone at the time of the incident. In an interview with ABC News, the girl, who was not identified because she is underage, said that she heard Mr. Martin ask, “ ‘What are you following me for?’ and the man said, ‘What are you doing here?’ Next thing I hear is somebody pushing, and somebody pushed Trayvon because the head set just fell. I called him again and he didn’t answer the phone.”

Could the apparent scuffle be enough to shield Mr. Zimmerman from charges, even if his decision to track Mr. Martin — despite police orders not to do so — triggered the confrontation? Perhaps — especially because of “Stand Your Ground” laws that all but invite gun owners to shoot first and ask questions later. But the Justice Department’s presence at least renews confidence that a credible investigation will take place.