MITO -- JCO Co. officials admitted in court on Monday they caused two workers' deaths through sloppy practices at the nuclear plant when it leaked radiation in 1999.

The former manager of the Tokai, Ibaraki Prefecture, plant, Kenzo Koshijima, added at the Mito District Court that he was guilty of causing the world's second-largest nuclear accident by approving the use of a metal bucket to treat radioactive materials.

Koshijima and five other officials, as well as JCO as a corporation, are accused of causing the deaths through negligence of Hisashi Ouchi, 35, and Masato Shinohara, 40, who died after the leak.

The firm expressed regret over the fatalities. JCO President Tomoyuki Inami, representing the company at the court, said, "No amount of repentance will be enough if you think about the loss of two precious lives."

In 1995, Koshijima sanctioned using a metal bucket -- against government rules. The following year, he drew up an illegal operation manual, prosecutors said. Koshijima and the other five plant officials admitted their failure to provide necessary safety education and supervision contributed to the two deaths. Many workers were only given a 15-minute briefing on safety in 1992, according to the indictment.

Prosecutors revealed that lax procedures were going on at the Tokai plant for over a decade.

"The possibility of radioactive leak is low, but if one does occur, workers will be definitely exposed to radiation," a JCO in-house report on its operations admitted as early as in 1992.

The nation's biggest nuclear accident occurred on Sept. 30, 1999, when Ouchi, Shinohara and Yutaka Yokokawa poured about seven times the company-set safety standard of uranium into the container solution.

Thousands of residents were forced to evacuate or remain in their homes as radiation levels dangerously soared in Tokai. Some 667 people were exposed in the incident.