For most Americans, the Fourth of July was a respite from the daily grind of work. But one group of laborers experienced a tragic, and deadly, holiday weekend.

Accidents killed five employees of fireworks companies across the country, including four who died after an explosion in North Carolina's Outer Banks.

A fifth victim, 19-year-old David Walker of Pulaski, Lawrence County, was struck by debris during a fireworks display in eastern Pennsylvania. He was an employee of Zambelli Fireworks, the famous New Castle company that produces more than 3,500 shows a year.

"The whole industry -- we're in mourning today," said Gregg Smith, safety program manager for the American Pyrotechnics Association, or APA. "This was not a good year."

Yet Mr. Smith and other proponents of the lucrative fireworks business argue that their work is well-regulated and heavily focused on safety.

"All companies do an expensive amount of training, both hands-on and in the classroom," said Stephen Vitale, president of Pyrotecnico, another New Castle company and the organizer of Saturday's 25-minute Three Rivers Regatta fireworks show.

"Safety is our top priority, as it is for Zambelli," he said. "They're a good company, and we're extremely saddened by their loss."

According to Mr. Smith's organization -- which is based in Bethesda, Md., and has about 250 members -- the United States has drastically increased its use of fireworks during the last several decades, jumping from 29 million pounds in 1976 to over 265.5 million pounds in 2007. Yet the number of fireworks-related injuries have fallen more than 90 percent.

Less than one-half of 1 percent of fireworks fail to function properly, Mr. Smith said.

He estimates that there were about 12,000 pyrotechnic displays over the holiday weekend, with the assistance of as many as 125,000 workers.

All those workers underwent thorough training, and their actions are heavily regulated by federal, state and local laws.

"Every year there's going to be accidents. I'm a realist," Mr. Smith said. "We're dealing with hazardous materials."

Yet the risk has been greatly reduced by safety precautions. Most pyrotechnicians only work part time. But they still face rigorous obstacles before they can participate in a major fireworks display.

Rules vary from state to state, but technicians must be at least 18 and attend a day-long course, often provided by a fireworks display company, like Zambelli.

The federal Bureau of Alcohol, Tobacco and Firearms performs a background check. A technician then becomes an apprentice, observing veteran workers during three to six outdoor fireworks shows.

Anyone who transports fireworks needs a commercial driver's license, with special permission to carry hazardous materials. Drivers must also receive additional background checks, including drug tests.

The majority of outdoor displays are operated by hand, Mr. Smith said. Technicians set up a rack of mortars and load the firework shells, with the fuse hanging over the top. The operator removes the safety cap, lights the fuse and sprints away, usually a distance of 25 feet.

The firework launches within three to five seconds.

Then the operator goes back in and lights another one, while a "spotter" stands back and watches for problems. Most crews include two to four people, and all wear safety helmets, goggles and ear protection.

Pittsburgh's Saturday display was coordinated by computer, with 22,000 fireworks shooting off from barges in the rivers, while the 12-person crew operated from a shelter.

In Pennsylvania, local governments are responsible for granting permits for fireworks displays. A permit costs $66 in Pittsburgh, but a company must also have a certificate of insurance, according to city fire Inspector Floyd "Skip" McCrea.

The company must also pay for an inspector to accompany the fireworks into the city and watch the unloading process. The delivery and unloading for Pittsburgh took place over three days, Inspector McCrea said.

Transportation is one of the most dangerous parts of a fireworks display.

The North Carolina blast came as workers were unloading fireworks, shaking homes and businesses across the southern end of Ocracoke Island and rattling residents and tourists. Dock master Robert Raborn was about 200 yards away from the truck and said the explosion was one of the loudest things he had ever heard.

Earl Woodham, an ATF spokesman, said agents determined the cause was accidental. The bureau will not investigate further, but a workplace safety agency such as the state Labor Department likely will try to find the specific cause, he said.

It was unclear if the fireworks display that killed Mr. Walker was operated electronically or by hand. Zambelli officials declined to discuss specifics, although they released a lengthy statement.

"Our heartfelt sympathy goes out to Mr. Walker's family and friends," the statement said. "Zambelli Fireworks will work closely with all of the local, state and federal agencies that have jurisdiction over our operations in order to reach an accurate conclusion to this tragic incident and to ensure that it does not happen again."

The accident occurred Saturday night in Quakertown's Memorial Park in Bucks County, near Philadelphia. According to county Fire Marshal Nicholas E. Rafferty, Mr. Walker was hit in the neck by wood when a shell exploded on the ground and destroyed a containment bunker.

Mr. Walker was pronounced dead at a nearby hospital.