Probe launched into killer explosion at Japanese chemical plant

Police said a total of 151 officers and investigators had begun an on-the-spot inspection at the plant, where workers had been refining chemicals to be used semiconductor production. "We started investigations this morning, mainly trying to find the cause of the explosion," said a spokesman for the Gunma Police Department. "But we can naturally think that there was something wrong with the chemical refining prodecures, which might have triggered the blast," he said.

The four killed were employees of the plant, while the injured included local residents, the spokesman said. "The number of injuries may increase as additional people may claim their injuries later in the day," the spokesman said.

The plant reportedly manufactured various kinds of hydroxylamine, which is used in the processing of semiconductors and as an ingredient in medicine and pesticides. While they are not toxic for humans, the substances are chemically unstable and prone to explode when heated to about 130 Celsius, news reports said.

Naoshi Imai, president of the chemical plant, apologised on Sunday for the explosion. "I am sorry that we caused this terrible accident," Imai told a news conference. "The cause will be found later, but I am afraid that hydroxylamine exploded itself." The blast caused power failures at more than 200 households late Saturday, but electricity had been restored by Sunday morning, officials said.