In one study, Dr. Strange showed people a film of a car accident in which five people, including a baby, were killed. The film was edited to remove segments of the accident. Then she tested the subjects 24 hours later on what they recalled. People were able to accurately describe what they had, in fact, seen, Dr. Strange said, but a significant number — 36 percent — also professed to have strong memories of parts of the crash that had actually not been shown to them.

“They are more likely to do that when they are upset about the event — if they are getting intrusive thoughts about it, or talking to other people about it,” she said.

A leading researcher in the field of witness memory, Elizabeth Loftus of the University of California, Irvine, said there was ample evidence that people found ways to plug holes in their recollections.

“If someone has gaps in their narrative, they can fill it in with lots of things,” she said. “Often they fill it with their own expectations, and certainly what they may hear from others.”

These are not the knowingly untrue or devious statements of people who are deliberately lying. False memories can be as persuasive as genuine ones, Dr. Loftus said: “When someone expresses it with detail and confidence and emotion, people are going to believe it.”

Said Dr. Strange, “It is surprising to the average person how quickly memories can be distorted.”

That was certainly Ms. Khalsa’s response.

“I feel totally embarrassed,” she said on Thursday, after having seen the video.

She now believes that she saw the initial encounter and then looked away, as she was on her bicycle. In that moment, the man began the attack, which lasted about three seconds until he was shot. “I didn’t see the civilian run or swing a hammer,” she said. “In my mind I assumed he was just standing there passively, and now is on the ground in handcuffs.”