The National Security Agency has kept secret since 2001 a finding by an agency historian that during the Tonkin Gulf episode, which helped precipitate the Vietnam War, N.S.A. officers deliberately distorted critical intelligence to cover up their mistakes, two people familiar with the historian's work say.
The historian's conclusion is the first serious accusation that communications intercepted by the N.S.A., the secretive eavesdropping and code-breaking agency, were falsified so that they made it look as if North Vietnam had attacked American destroyers on Aug. 4, 1964, two days after a previous clash. President Lyndon B. Johnson cited the supposed attack to persuade Congress to authorize broad military action in Vietnam, but most historians have concluded in recent years that there was no second attack.

The N.S.A. historian, Robert J. Hanyok, found a pattern of translation mistakes that went uncorrected, altered intercept times and selective citation of intelligence that persuaded him that midlevel agency officers had deliberately skewed the evidence.

Mr. Hanyok concluded that they had done it not out of any political motive but to cover up earlier errors, and that top N.S.A. and defense officials and Johnson neither knew about nor condoned the deception.

Mr. Hanyok's findings were published nearly five years ago in a classified in-house journal, and starting in 2002 he and other government historians argued that it should be made public. But their effort was rebuffed by higher-level agency policymakers, who by the next year were fearful that it might prompt uncomfortable comparisons with the flawed intelligence used to justify the war in Iraq, according to an intelligence official familiar with some internal discussions of the matter.