He was a man for whom the term muckraker was coined, a crusading journalist and novelist who never hesitated to expose scandal at the highest levels of government and business. But now the integrity of Pulitzer Prize-winning writer Upton Sinclair is being questioned 38 years after his death because of the discovery of a letter he wrote in 1929.

Quotes from the letter in recent news reports make it seem that the man who exposed the horrors of the meat-packing industry in the 1906 book "The Jungle" covered up a confession from a defense lawyer that famous anarchists Nicola Sacco and Bartolomeo Vanzetti were guilty of the murders for which they were executed. Many people thought the two were innocent and prosecuted for political reasons.

But Sinclair's defenders say key quotes from the letter exonerating him from that charge were ignored and that instead of covering up his doubts about the case, he devoted a whole novel to them, "Boston." They say he is being smeared 38 years after his death.

In the month since the Los Angeles Times article and other articles on the letter appeared, conservatives have seized on the letter as proof of liberal perfidy. Columnist Jonah Goldberg called Sinclair a liar and said telling the truth would have cost him too many readers.

He linked Sinclair's support for Sacco and Vanzetti to later discredited causes of the Cold War like the Rosenberg and Alger Hiss cases.

But Goldberg might have been better served if he had read the entire letter instead of the excerpts printed in the Times or if he had access to a soon-to-be published biography by Anthony Arthur called "Radical Innocent: Upton Sinclair."

In a copy of the full letter made available to Reuters, Sinclair says that soon after he talked to Moore he began to have doubts about him. "I realized certain facts about Fred Moore. I had heard that he was using drugs. I knew that he had parted from the defense committee after the bitterest of quarrels. ... Moore admitted to me that the men themselves, had never admitted their guilt to him; and I began to wonder whether his present attitude and conclusions might not be the result of his brooding on his wrongs."

Sinclair questioned Moore's former wife who worked with the lawyer on the case, and she "expressed the greatest surprise" saying he had not expressed thoughts that the men were guilty before.

In the letter, he also vowed "Boston" would tell all sides and "I would take my stand on the point that the men had not been proved guilty and that their trial had not been fair."

Arthur, who provided advance excerpts of the biography to Reuters, says that in other letters Sinclair quotes Moore as not even being sure both men were guilty.

"Moore said neither man ever admitted it to him, but he was certain of Sacco's guilt and fairly sure of Vanzetti's knowledge of the crime if not his complicity in it," Arthur wrote in the biography which will be published in June by Random House.

He added, "His knowledge had not prevented Moore from doing whatever he could to save the two men, perhaps including illegal activities. The entire legal system, was corrupt, Moore insisted, assuring Sinclair that, 'There is no criminal lawyer who has attained to fame in America except by inventing alibis and hiring witnesses. There is no other way to be a great criminal lawyer in America."' ...