Japanese leaders rushed to cover up the germ warfare because they knew it violated international law

Takao Matsunaga, history professor

The historians were testifying before the Tokyo District Court as witnesses for a group of Chinese people demanding reparations for the deaths of relatives.

The Chinese plaintiffs claimed their relatives were killed in biological experiments and other acts of brutality carried out by Japan during World War II.

The court heard last month how the military ''bombed'' a Chinese city with bubonic plague-carrying fleas, triggering a serious outbreak of the disease.

The lawsuit claims at least 2,100 people were killed in biological experiments by Japan's notorious Unit 731.

Although some Japanese veterans have confessed to war crimes, the Japanese government has refused to make a written apology to China for atrocities committed before and during the war.

'Deal with US'

The Japanese historians testified that the full extent of the germ attacks remained unknown partly because the United States made a deal with the Japanese not to prosecute those involved in exchange for the scientific data.

Hirohito died in 1989 and was succeeded by his son

According to the historians, the US was concerned that the information might fall into the hands of Russia.

The witnesses said Japan's military headquarters ordered the Unit 731 to destroy evidence when the United States dropped the second atomic bomb on Nagasaki.

Unit leaders later landed top jobs at universities and pharmaceutical companies, keeping their identities secret.

Japan acknowledges that Unit 731 existed, but has refused to confirm its activities.

The trial in Tokyo District Court is expected to continue for several months.

Hirohito died in 1989 and was succeeded by his son, the current Emperor Akihito.