Powell, who had lived for many years in San Francisco, died in the city Monday of complications from pneumonia, son John S. Powell told Wednesday's New York Times.

U.S. prosecutors put Powell on trial in 1959 on a rare charge of sedition, after he authored articles for his Shanghai publication The China Monthly Review asserting that U.S. military had employed germ warfare against North Korea, using methods they had allegedly learned from the defeated Japanese army.

Powell wrote that a secret unit of the occupying Japanese army known as Unit 731 had carried out large-scale biological warfare attacks on Chinese soldiers and civilians during World War II, killing hundreds of thousands.

The U.S. government eventually dropped all charges against Powell, but his case dragged on for five years and became a cause celebre.

Afterwards, unable to continue his journalistic career, he made a successful career of buying and restoring Victorian mansions in San Francisco and operating an antiques business, the Times said.