After Baltimore police officers fatally shot a man in the Douglass Homes housing project last week, the accounts of what happened were nearly as great in number as the people who said they saw it.
Police said 25-year-old Donte Bennett took out a gun before officers shot him. One resident said she saw the man's hands in the air; another said he was on his back with officers holding his wrists.
Bennett's mother called for an outside review of the incident, and police say they have opened an internal investigation. Experts say any such inquiry would have to sift through conflicting reports from witnesses whose recollections can be colored by vantage point and life experience.
Susan Courtney, a professor at the Johns Hopkins University's Department of Psychological and Brain Sciences, said memories are rarely precise. "We only remember bits and pieces of what we see," said Courtney, who specializes in working memory. "Then we create a plausible explanation of what happens, based on our assumptions."
According to an August 2011 survey conducted by researchers at the University of Illinois, two-thirds of 1,500 respondents erroneously believe that memory works like a video camera, with people able to accurately recall information during a later review.
Courtney said people's frame of mind, vantage point and what they were paying attention to at the time are among the issues that will effect recollection of an event.
For example, an individual may see a dark figure entering a vehicle and later observe a dark figure exiting it. The person may be convinced that this is the same individual, but someone else viewing the situation from another perspective may observe two different figures.
"Every time we recall something, we re-evaluate it," she said. "The memories become more distorted over time, but you become more convinced that your version is accurate."
A University of Houston law professor writing for The New York Times in September 2011 argued that faulty eyewitness testimony has led to the wrongful conviction of many innocent African-American men. Factors that contributed to this were police suggestion and the amount of fear the witnesses displayed when making identifications.
"If we have biases, we can reconstruct a scenario and believe that really happened," Courtney said.
Ultimately, she argued, it's the responsibility of the police to be thorough in their witness interviews and carefully correlate the statements with the physical evidence.
"Every situation is different, every crime is different," said Sgt. Eric Kowalczyk, a Baltimore police spokesman. "It's at the officer's discretion to determine what belongs in the report."
A previous version of this story incorrectly described Susan Courtney's role at Johns Hopkins.