What Force Science Still Teaches About BART Case, Despite Court Ruling

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In the nation’s highest-profile case of weapons confusion, the California Court of Appeal has ruled that a jury verdict of involuntary manslaughter was reasonable and a two-year prison sentence was warranted for former officer Johannes Mehserle, who swore that he thought he was deploying his Taser when he actually drew his pistol and fatally shot a struggling suspect.

Mehserle’s defense team, including Dr. Bill Lewinski, executive director of the Force Science Institute, had hoped the conviction would be overturned on grounds that the shooting was a profound but innocent mistake. Lewinski had testified at trial that compelling psychological stress phenomena well known in the world of behavioral science had driven the officer’s unfortunate action, not recklessness.

But the three-judge appellate panel saw the matter differently, unanimously affirming last month that the jury was justified in concluding that Mehserle’s fatal deed constituted “criminal negligence” and “conscious indifference to the consequences”—“not a mere mistake.” [The decision can be read in full by clicking here]

“I am disappointed that the human performance dynamics of this case, which play a critical role in thoroughly and accurately understanding what happened during this incident, were not taken strongly enough into account” Lewinski told Force Science News. “There is still much work to be done to help judges and jurors comprehend the huge gap between the deliberative way decisions are made in a courtroom and the way officers often must make them on the street, in split-seconds and under extraordinary stress.”

Since Mehserle’s trial in 2010, Lewinski has been dissecting the shooting from a behavioral science standpoint as part of FSI’s popular five-day certification course in Force Science Analysis.

That presentation will continue essentially unaltered despite the appellate decision, he says, because “this case is a classic illustration of how psychological and physiological factors can impact decision-making, creating results that are very different from what people judging an officer’s actions might expect and the officer intended.”

Lewinski’s analysis takes some 90 minutes in class. Here, in condensed form, are some of the core concepts he discusses.

SHOT FIRED. At the time of the shooting, Mehserle was a young officer with the Bay Area Rapid Transit (BART) District, patrolling public transportation lines in the greater San Francisco region. In the early minutes of 2009, he and other officers responded to a fistfight involving at least 10 males on a BART train. At a stop in Oakland, one of the alleged combatants (and a frequent lawbreaker), 22-year-old Oscar Grant III, was extracted from one of the cars and proned out on the station platform.

While several transit officers tried to control a raucous and hostile crowd of some 300 New Year’s Eve celebrants on the train and platform, including rival gang members shouting threats to kill each other, Mehserle and one other officer struggled to handcuff Grant.

The suspect stubbornly resisted Mehserle pulling his right hand out from under his chest. Mehserle later testified that he decided to tase Grant to get compliance. He announced aloud his intention. As he released Grant’s arm and started to rise, Grant lifted up slightly, turned to his right and moved his hand, first down toward his waistband then toward his front pocket, “as if he were grabbing for something.”

Earlier in his shift, Mehserle had witnessed the arrest of another offender who had a gun concealed in a front pocket. He himself on two other occasions had taken guns from suspects’ pockets. Grant later proved to be unarmed, “but in context Mehserle instantly perceived Grant’s movement as potentially life-threatening, requiring an urgent response,” says Lewinski.

Under this pressure, instead of pulling his X26 Taser, Mehserle grabbed his .40-cal. Sig Sauer 226 and mortally wounded the suspect with one round to the back.


A variety of “compelling elements” drove Mehserle’s mistaken action, Lewinski contends. “A sense of urgency, time compression, narrowed focus of attention, an automatic programmed response—all conspired to create and reinforce a dominant, fateful phenomenon called a ‘slip-and-capture error,’ ” he says.

“This is a mistake that’s made when your intended behavior, in effect, ‘slips off’ the path you wanted it to go because it is ‘captured’ by a stronger response and sent in a different direction.”

In Mehserle’s case, he’d carried a Taser only rarely on duty and had drawn it perhaps a dozen times or so, including practice in training and never under stressful, time-pressured circumstances. In contrast, he’d been drawing his sidearm an estimated 50 times a week as practice since graduating from the academy about two years earlier, building up through repetition what Lewinski calls “a strong automatic motor program” so he could draw quickly without having to consciously think about each mechanical step in the process—usually a highly desirable skill in threatening circumstances.

Under extreme time pressure to manage the perceived threat from Oscar Grant, Mehserle’s intention to draw his Taser “slipped off his agenda, so to speak,” Lewinski believes, “and was captured and completed by the more well-rehearsed motor program of going to the location of his gun.”

There is no doubt, Lewinski insists, that Mehserle was unaware that he’d grabbed the wrong weapon until after the shot was fired. Enhanced on-scene video played and critiqued in the certification class shows Mehserle tugging to release his gun from its holster with the kind of cant he’d use to draw his Taser; his gun didn’t have an external safety but his thumb can be seen moving as if to activate a Taser switch; his eyes are focused on Grant’s back where he’d want to place the darts; he rises to get a better spread before pulling the trigger; he’s visibly startled when the gunshot goes off, exclaiming “Oh shit!” or “Oh God, I shot him!”; and so on.

“At least eight different cameras, including civilian cell phones, recorded different phases of the incident,” Lewinski says, “and the video is very supportive of the interpretation that the shooting was an honest mistake.”

Aberrational errors akin to Mehserle’s are well-documented in other professions, as well as in everyday civilian life, Lewinski points out. He cites examples of experienced, highly trained pilots who inadvertently activate the wrong controls or initiate the wrong maneuvers at critical moments and crash airplanes and drivers who floorboard their accelerator when they think they’re tromping on the brake.

“In law enforcement,” he says, “we had instances of officers dying in squad car crashes after antilock brakes were introduced. In the crisis of an unexpected skid, they’d revert to pumping the brakes instead of steadily engaging them, and thereby fail to regain control of the car. Under stress, they unconsciously resorted to their old and more practiced programming.”

He recalls a minor slip-and-capture of his own that occurred, ironically, shortly after he left a conference with attorneys on the Mehserle case. A sudden cloudburst hit, but instead of reaching for the windshield wiper controls where they were located on the rental car he was driving he instinctively reached for where they would be on his personal vehicle, a different make and model.

“I had time to realize my mistake and correct it,” he says, “but in Mehserle’s situation, he didn’t have that luxury of time. The fact that errors of the magnitude of his are relatively rare does not mean they can’t occur. They can and do. In fact, they are very common in law enforcement and in our everyday life but usually have minimal consequences and rarely have any substantive or lasting impact.”


The appellate court declared that Mehserle had ample reason to recognize that he had deployed the wrong weapon: His handgun was three times heavier than his Taser, a different color, with no on/off or safety switch, no laser sight, and a holster located in a different place and with a different release mechanism. “A reasonable jury could conclude that a reasonably prudent person could distinguish between the two weapons,” the jurists wrote.

If Mehserle had had more time, under less urgent circumstances, he may well have recognized his mistake, Lewinski argues. “But only approximately two seconds elapsed between the time he rose up to draw and the time his fatal shot hit Oscar Grant.”

In that tiny timeframe, his sole focus would have been on the threat he felt the suspect posed. His brain’s reception of other stimuli, like the feel, the color, and the details of the weapon in his hand, would have been blocked out by what behavioral scientists call “inattentional blindness” or “selective attention,” Lewinski explains. This would be assisted by what is called the “Colavita Effect”, which is the brain’s primary reliance on visual cues under this level of threat and time compression.

“In a very brief, very intense, very high-consequence encounter like Mehserle was in, the brain doesn’t have the capacity to be aware of everything in the surrounding environment. It narrows its focus to concentrate only on what it perceives as necessary for surviving—in this case, resolving the threat. The rest, it simply doesn’t—can’t—pay immediate attention to. Mehserle’s behavior would have been unconscious and automatic.”

The appellate court dismissed inattentional blindness as “not something suffered by a reasonably prudent person.” That, Lewinski declares, is blatantly inaccurate. “This performance issue is well-understood and thoroughly documented in scientific literature,” he says, “and has been repeatedly confirmed in research by the Force Science Institute and other reliable sources. Attentional narrowing, a primary reliance on visual cues and inattentional blindness are human reactions that occur under stress and can affect anyone beyond their immediate control.”


A major problem with educating judges, jurors, and prosecutors about cases like Mehserle’s is that few of them have likely ever been forced into decision-making like his, Lewinski says. “They simply don’t experience this type of incident, so they don’t instinctively identify with the dynamics involved.

“The expectation in the judicial system is that decisions are made and actions are taken coolly, logically, and rationally. The Mehserle ruling assumes he was consciously aware of and processing everything going on. But under stress, the brain operates in an entirely different fashion and consequently, at times performance will be different than expected.”


At the time of the shooting, BART officers were not permanently assigned a particular Taser. Indeed, the agency had four different holster configurations and belt placement for the weapon and an officer could end up with any version on a given shift—a fact that Lewinski believes may have contributed to Mehserle’s slip-and-capture error.

Mehserle’s rig on that night was positioned just to the front left of center on his waist, requiring a dominant-hand cross-draw. It was perhaps a foot away from his sidearm.

Now BART mandates that its Tasers be carried on the nondominant side and drawn with the nondominant hand, an arrangement Lewinski supports. “That may not eliminate the risk of a slip-and-capture error entirely,” he says, “but it should lessen the possibility.”

Once drawn, the Taser can be switched to the dominant side. “However as good trainers point out, officers should be competent in using their defensive weapons with either hand under any circumstances,” he says.


“Through modern training, investigators of officer-involved shootings are coming to better understand the behavioral dynamics of force encounters that are not always obvious or easily explained,” Lewinski says. “But as the Mehserle case shows, we still face a formidable challenge of how to better bring the science of human performance into the courtroom to more realistically guide people who are judging officers’ actions.”

After the appellate decision last month, Mehserle’s attorney, Michael Rains, said he would appeal the case to California’s Supreme Court. Mehserle, meanwhile, has been paroled after serving 11 months of his sentence. He fights now to clear his name and professional reputation.

[For more information on the BART case, see an excellent PowerPoint presentation on that and other weapons-confusion incidents created by Force Science Analyst Greg Meyer and posted by Americans for Effective Law Enforcement. Click here for a copy of the presentation.]

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