Recently released U.S. government documents have thrown new light on a celebrated resident of Japan's death row, Sadamichi Hirasawa, who has been awaiting execution for 30 years for his conviction in a 1948 mass murder at a Tokyo bank.

The papers tend to support his family's assertion that the murders were probably committed by a veteran of a secret chemical and germ warfare unit of the disbanded Japanese Imperial Army.

The papers detail the role in the murder investigation by the U.S. military occupation headquarters in postwar Japan and were submitted recently to a Tokyo court to support a petition for a new trial for Hirasawa, now 93, the oldest resident on Japan's death row.

After the war, the United States granted members of the Japanese unit immunity from war crimes prosecution in return for data from their experiments. This did not become generally known in Japan until the 1970s, when newspapers here wrote extensively on the subject.

Hirasawa's supporters have contended that the Americans, fearful of exposure of the unit's operations and their decision not to prosecute, may have ordered the Japanese police to halt investigations of the unit. But the documents offer no support of that assertion.

Hirasawa, now in failing health, was convicted of the Imperial Bank incident of Jan. 26, 1948. On that afternoon, a man posing as a health inspector entered a Tokyo branch of the bank and tricked 16 employes into drinking poison by telling them it was medicine that would prevent an epidemic. The man fled with money from a safe as the employes lay writhing on the floor. Twelve of them died.

Hirasawa was arrested six months later and confessed. He quickly recanted his confession, however, citing police coercion. Nonetheless, he was convicted and sentenced to death. In 1955 the Japanese supreme court upheld his conviction and he entered death row, but his execution has never been set.

Outside prison, a support group has been campaigning for decades for his freedom. One of their key contentions is that prosecutors never explained how Hirasawa, a professional artist, could have obtained the poison and been so well-versed in its dosage and effects and the psychology of inducing the people to drink it.

The U.S. documents were obtained by writer William Triplett. His book about the bank crime and the investigation, "Flowering of the Bamboo," is to be released this fall by Woodbine House, a Washington book publisher.

Two documents detail a Tokyo meeting attended by U.S. occupation officials and the chief Japanese detective in the case, Jiro Fujita. It took place on March 11, 1948, several months before Hirasawa's arrest.

According to the documents, Fujita told the group that the best police leads were people who had been linked with a secret Japanese Army chemical laboratory, "where experiments were perfected in the use of prussic acid, the poison used by the Teikoku Imperial Bank robber."

Techniques for using the poison had been published in an Army pamphlet, and the Teikoku robber used the identical method, Fujita told the group.