WASHINGTON -- On a September afternoon in 1970, Air Force Lt. Tom Stump flew low over Laos in his A-1 attack plane, supporting a U.S. ground operation against a North Vietnamese stronghold.
Stump unloaded bomblets on the enemy troops while other U.S. planes dropped canisters of gas. Crackling over his radio came the sounds of American troops choking and coughing.
"Those guys got a pretty good dose of the stuff," he recalled. "[The planes] went right in over our guys."
That incident is the focus of a report that the gas was sarin -- the substance that killed 12 people on a Tokyo subway in 1995 -- and that it was used against a group of U.S. defectors and that it wounded American troops positioned near the enemy.
The Pentagon is investigating the claim, and the Vietnam Veterans of America has called for a congressional investigation. The Senate Veterans Affairs Committee will determine whether to hold hearings, a spokesman said.
But classified documents and interviews with some participants in the operation have raised questions about the accuracy of the story, reported on CNN and in Time magazine last week.
The chairman of the Joint Chiefs of Staff at the time, who CNN said had confirmed the story, now flatly denies he did so.
A former Green Beret captain who was mentioned in the report says he doesn't believe sarin was used.
And a former Green Beret lieutenant who was quoted as saying sarin was part of their arsenal now says it was the show's producer who told him nerve gas was used.
A previously top-secret report of the 1970 incident, conducted immediately afterward and just released by the Pentagon, makes no mention of sarin.
Pentagon weapons experts say no lethal gas was used in Vietnam; U.S. policy forbade troops to initiate the use of nerve gas.
Rather, it was tear gas that was used that day, the Pentagon and pilots who flew the missions say. If it had been nerve gas, one general said, at least some of the U.S. soldiers would have been killed.
CNN is sticking by its story. Though some of its named sources have now distanced themselves from it, the report is also based on unnamed Special Forces, Marine and Air Force veterans, said April Oliver, the show's producer.
"They are very, very scared of [being charged with] war crimes," she said, noting that the story followed an eight-month Time/CNN investigation. "This big Pentagon inquiry, we welcome that."
Operation Tailwind was a four-day operation in Laos that began with Green Beret troops supported by Montagnards, a tribe allied with U.S. forces against the North Vietnamese.
The targeted village, CNN and Time reported, included a group of U.S. soldiers who had defected to the enemy and who might, it was feared, leak military secrets.
Softened up with sarin
One night, CNN and Time reported, sarin gas was dropped to "prepare the village for the attack."
After the firefight the next day, U.S. forces moved to a landing zone, where they were to be picked up by helicopters.
All 16 of the Green Berets had been wounded. There were 144 enemy killed, with 288 more killed by air attacks.
The CNN report, citing "military officials," said Tailwind held "two of the military's top secrets": the use of sarin and the targeting of American defectors. In addition, the report said, the nerve gas not only killed Vietnamese enemy troops but also injured some U.S. troops nearby.
CNN also said retired Adm. Thomas Moorer, chairman of the Joint Chiefs of Staff at the time, "confirmed that nerve gas was used in Tailwind."
But Moorer told the Associated Press afterward: "Whether they had sarin, you can't prove it by me either way. There were rumors that this gas had been used."
Oliver said Moorer confirmed both the use of sarin and the targeting of American defectors.
"I found Admiral Moorer to be a distinguished, thoughtful person," Oliver said. "I'm sure he's under a great deal of pressure now by his former colleagues."
Also interviewed for the story was former Capt. Eugene McCarley, who was commanding Green Beret troops.
"They didn't quote me much; I wasn't very sympathetic to what they were doing," McCarley said. "It's 90 percent lies. I'm wondering if I was on a different Operation Tailwind than the one CNN is talking about."
Oliver said McCarley told her that "very possibly" a lethal nerve gas was used.
"I said, 'Well, it's possible,' " McCarley said. "But I continued: It was never used by any of my forces, and it was never used in
Another Green Beret soldier who took part in Tailwind, Lt. Robert Van Buskirk, talks on the program and in the related Time article about nerve gas and of seeing two Caucasians at the camp.
The report said he saw the two of them run into a hole and that he threw in a grenade, which likely killed them.
In an interview, Van Buskirk says he recalls seeing Caucasians but said he had repressed the memory for years. Oliver's questioning, he said, helped revive those memories. "I let it go, CNN brought it back," he said.