The U.S. National Security Agency has kept secret since 2001 a finding by an agency historian that NSA officers deliberately distorted critical intelligence during the Tonkin Gulf episode that helped precipitate the Vietnam War, according to two people familiar with the historian’s work.

The historian’s conclusion is the first serious accusation that communications intercepted by the NSA, 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.

Any of this sound familiar?

The NSA historian, Robert 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.

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

The research by the NSA historian was detailed four years ago in an in-house article. It remains classified, in part because agency officials feared its release 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.

You wouldn’t want the Iraq War to make some of our politicians uncomfortable.

The supposed second North Vietnamese attack, on the U.S. destroyers Maddox and C. Turner Joy, played an outsize role in history. Johnson responded by ordering retaliatory airstrikes on North Vietnamese targets and used the event to persuade Congress to pass the Gulf of Tonkin resolution on Aug. 7, 1964.

It authorized the president to “to take all necessary steps, including the use of armed force” to defend South Vietnam and its neighbors and was used both by Johnson and President Richard Nixon to justify escalating the war, in which 58,226 Americans and more than one million Vietnamese died.