The allegations about Operation Tailwind, which was the subject of a high-profile CNN television documentary and an accompanying Time magazine article, have been increasingly challenged in the two weeks since they were aired.
They have also sown serious discord between the two arms of the Time- Warner empire.
In the programme, former servicemen testified on camera that they had seen mysterious gas canisters loaded on to a plane, the effect of which was to destroy the nerve functions of soldiers on the ground, and that special White House authorisation had been required for the operation.
The episode was said to have taken place in Laos in 1970 during the presidency of Richard Nixon.
Doubts about the claims surfaced immediately after the CNN documentary was broadcast.
Retired Admiral Thomas Moorer, chairman of the joint chiefs of staff at the time, insisted he had been misquoted and demanded a retraction. Senior journalists at Time also questioned the veracity of the allegations, loudly enough for the magazine to add a question-mark to the headline of its companion article.
Much rested on the success of the project, however, which was a heavily promoted collaboration between CNN and Time magazine.
CNN programme-makers defended themselves vigorously. Reporters were said to have conducted more than 200 interviews for the project and to have carried out rigorous checks.
The Pentagon responded within hours of the broadcast, saying it had no knowledge of such an operation. Within 24 hours, however, it had instituted an inquiry, which is due to report in two weeks' time.
Successive military spokesmen suggested that if gas was used, it was most likely tear gas.
A rash of fierce press articles followed in which former officials and servicemen questioning the soundness of the allegations and the credibility of the chief accuser, Robert van Buskirk.
As the doubts multiplied, CNN broadcast a clarification of Admiral Moorer's words, but no retraction. It also adjusted its version to say the gas had been used as a last resort to rescue US troops from an ambush, dropping the most eye-catching, but shocking charge that the attack was targeted at US defectors.
Meanwhile, the mood at Time was mutinous, with journalists still questioning the report - and, by implication, the standards of their television colleagues and the wisdom of the Time/CNN collaboration.Reuse content