A Manassas woman's serious burn during Saturday night's elaborate fireworks display at the Mall apparently was caused by a rain-dampened shell that was blown over the crowd by the wind, then ignited on the ground, according to the designer of the show.

Tom Zarlingo, who orchestrated the $95,000 display for Vitale Fireworks Display Co. of New Castle, Pa., said yesterday that moisture from a brief shower may have dampened the fuse of the shell, causing it to ignite on the ground rather than at the height of its trajectory.

That account contradicts earlier reports that Kathy Eldreth, 30, was injured by flaming debris from a shell that had exploded in the air. Eldreth was taken by helicopter to the Washington Hospital Center, where she was treated for a fist-sized, third-degree burn on her back and released.

"It isn't unusual to have a shell break low," Zarlingo said. "It is unusual to have a shell fall and then go off."

The National Park Service is investigating the mishap and will issue a report in a few days, officials said.

Park Service officials are rethinking safety precautions for the annual fireworks display, which this year attracted a crowd estimated at 500,000. One possibility is the expansion of the so-called safety zone, the fenced-off area east of the Reflecting Pool from which the rockets are launched, according to spokeswoman Sandra Alley.

About 4,500 rockets were launched Saturday night, some weighing as much as 25 pounds. There were nearly twice the number as last year, and the safety fences had been expanded accordingly, by 500 feet to more than 1,000 feet, officials said.

U.S. Park Police Maj. Robert Hines, who is handling the investigation, said that some unexploded shells were found in the fenced-off area after Saturday night's display. Eldreth was 75 feet from the boundary when one apparently fell outside the fence and landed near her, Hines said.

Hines said she may have been injured by a "secondary" charge, borne aloft as part of a cluster of small shells. In any event, according to Zarlingo, "there must have been some wind that blew it" outside the safety zone. He said that ignition of the shell almost certainly was delayed by moisture from the day's intermittent rain.

In an interview last week, Zarlingo, who is an obstetrician at Case Western Reserve University when he is not designing fireworks displays, had expressed concern about the possibility that rain could interfere with his fuses. In Pennsylvania recently, a shell exploded on the ground near Zarlingo and caused a wound on his scalp that required 16 stitches to close.

Despite the larger launching area, as many as 25 people were treated by medics at the scene for minor burns from hot ash and smoldering cardboard debris, according to initial reports. Hines, however, put the number at "half a dozen" and played down the significance of the injuries, noting that only Eldreth required treatment at a hospital.

"It's not that big of a deal," he said. "We've had people hurt at others." Officials said, however, that this was the first time in recent memory that anyone had been injured seriously enough to be taken to a hospital as a result of the official fireworks display.