It's the question that surfaces from the conflicting accounts given by former Nebraska Sen. Bob Kerrey and another man of a bloody episode involving the killing of civilians during the Vietnam War.
The answer, say experts in the science of memory, may be that neither is.
Both men may be honestly recalling events that occurred 32 years ago on a horrifying night in the Mekong Delta.
But memories are often wrong when they are formed, and tend to shift each time they are retrieved, considered, discussed and tucked away.
"That people would have different memories is possible," says Dr. Craig Stark, a memory researcher at the University of California, San Diego. "What we often remember from episodes is not always the original event, but reconstructions of the event.
"The analogy of a video recorder going around and recording what happens is not the way memory works," Stark says. "We always have bits and pieces of events and try to reconstruct and retrieve them."
The story of the raid on Thanh Phong, a tiny village in South Vietnam, on Feb. 25, 1969, was the subject of an article April 29 in the New York Times Magazine and Tuesday on CBS' "60 Minutes II." Two former Navy SEALS - Kerrey and former squad member Gerhard Klann - give starkly contrasting versions of an event that both say has tormented them in the years since the war.
Kerrey, at the time a young lieutenant who led six Navy commandos into an area that was heavily occupied by Viet Cong, says he gave the order to open fire when his squad was fired upon. He says the men shot blindly into the night, discovering later that they had killed about 14 women and children.
Klann, who was under Kerrey's command, says the Americans did not return fire, but were ordered by Kerrey to round up and execute the villagers. He said the women and children were shot at close range outside the small cluster of huts where they lived.
Other veterans of the squad insist in a joint statement that they "received fire and returned fire." No order, they say, was given to execute women and children. But reporters interviewed a Vietnamese woman who says she witnessed the Americans rounding up and shooting the villagers.
Dr. Barry Gordon, a behavioral neurologist at the Johns Hopkins School of Medicine, says it is common for witnesses or participants in the same event to disagree on essential elements.
"It happens all the time," he says. "Perhaps one reason is that we're talking about events of 25 or more years ago. It's well known that for important events, people will have different perspectives at the time, and their perspectives change over time."
Almost anyone's memory can shift, he says. Distortions - involving the central truth or the details of an incident - are made by people whether they are perpetrators or victims of violent acts, by purported heroes and bit players, and by witnesses to events both traumatic and mundane.
Veterans of World War II battles have written accounts that are disputed by other people who were present. Victims of muggings have recalled events quite differently than did passers-by.
There are many reasons for this, Gordon says. As events happen, people may see them differently depending on where they are standing, what role they played or what emotions they carried at the time. Their memory of an event can be incomplete and contradict that of others.
As time passes, people sometimes forget whether they actually saw something or incorporated pieces of someone else's account into their own.
According to Gordon, people also experience "memory drift." As someone remembers an event over time, details around the edges of a memory tend to change. Eventually, the picture begins to take on a new cast.
Dr. Jason Brandt, a Hopkins neuropsychologist, says people also combine memories. Over time, a person might shuffle details of separate events that occurred around the same time or had common themes.
People also start to insert pieces of dreams into their memory, he says.
Researchers acknowledge that people lie to protect their reputations or even to talk themselves into believing a more palatable version of events. But they often distort memories without knowing it.
It remains unclear whether people are more likely to forget painful or benign events, says Brandt. Some experts argue that people cannot forget the worst things. A more Freudian view is that people subconsciously bury the worst memories to protect themselves from pain.
"There is not real good evidence that emotionally charged material is processed in a fundamentally different way," Brandt says. "The same rules of memory apply."
In a mundane example of memory distortion, adults revisiting their elementary schools are often surprised to see that the buildings don't look at all they way they remember.
But more extreme cases abound: Holocaust survivors who witnessed the same slaughters or survived the same death camps often have different memories, Brandt says. One may remember a person being shot; others may insist the victim was hanged.
"There are many cases in which there is no clear motivation for people to make things up," Brandt says. "These were very public events, and people who experienced the same thing had very different recollections."
In one study, researchers at the University of California, San Diego interviewed people a week after the verdict in the O.J. Simpson trial - asking them where they were when the news broke.
When the people were asked the same question a year later, many changed their stories. Some who had originally heard the news on the radio later recalled seeing it on television; some who first reported they were at home later said they were visiting a friend.
"Their memory was horrible," says Stark. "They invent details and they don't know they are doing it."
Researchers have documented cases in which people who didn't do anything wrong eventually came to believe they committed heinous acts. For instance, some parents whose children falsely accused them of sexual abuse have come to believe in their guilt, says Gordon.
Without convincing physical evidence, the truth of events such as the tragedy at Thanh Phong may be impossible to know.
"In essence, how do you judge the credibility of one witness over the other?" Gordon asks.
Depending on how one looks at the story, "Only one can be right, both can be wrong and the one who can be right doesn't have to be completely right," he says.