Two people were killed and two others were injured in an explosion during a fireworks festival in the town of Sanyo, Yamaguchi Prefecture, on Saturday evening, local police and firefighters said.
The four men were in charge of setting the fireworks off, they said. Investigators suspect that fireworks they were trying to launch accidentally exploded on the ground. The identities of the four were not immediately known.
The explosion occurred at 8:30 p.m., shortly before the two-hour display was due to end and after about 4,000 fireworks had been detonated.
The fireworks event, held at the Habu fishing port in the town, was attended by about 5,000 people.
None of the onlookers were injured as the accident took place at the fireworks launch site, on the tip of a dike about 100 meters from the port and off limits to the viewers.
The event is held annually as a prelude to a local festival. It is popular not just among locals but visitors from outside the prefecture as a fireworks event is rare at this time of the year, according to town officials.