Unintended Firearms Discharges in Law Enforcement – Principle and Contributory Causes; Part 4 of 7

Inspector (Ret) Chris Butler

Welcome back to Part 4 of our study into the causes of unintended discharges in law enforcement. If you have been reading along as we have explored this issue, you will recall that in Part 3 we took a look at the research into the human kinetics regarding sympathetic muscle contraction. If you have missed the previous articles on this subject, its important to take the time to review them as I am building upon concepts as we move along in this series.

This week, we are going to continue to wade through the science and research that pertain directly to the human factors that lead to unintended discharges.

Let’s start with a sudden and unanticipated loss of balance. What happens when an officer, with their gun in their hand, losses their balance during a confrontation with an offender or during a foot pursuit?

Incident: Several undercover drug officers were conducting a search warrant for cocaine at a two-story, single family residence in a large municipal city. The house had been searched and secured and the officers were walking down the hallway of the second floor when one of the officers noticed white stippling particles on the hallway carpet. Looking above them, the officer observed a wooden attic access that appeared to have been recently moved. In order to reach the attic access, the officers obtained a plastic laundry hamper from one of the nearby bedrooms and placed it under the access point. One officer climbed up onto the hamper while two other officers held onto to it. The officer, once on top of the hamper, removed his Glock sidearm and with his support hand pushed up on the wooden attic hatch. The counter-pressure of pushing up was enough force to cause the plastic laundry hamper to suddenly collapse; the officer standing on top now finding himself free falling through the air. Regrettably, the officer had his finger inside the trigger guard and upon the sudden loss of balance, his weapon discharged. Very fortunately, in this case, no other officers were injured and the round from his weapon entered the ceiling and lodged inside the attic area.

Unintended Firearm Discharge – Involuntary Muscle Contraction Caused by Sudden Loss of Balance

Another possible cause, which has been found to lead directly to involuntary muscular contractions (and thereby, weapon discharges), is a sudden loss of balance. When balance is unexpectedly disturbed, a series of rapid involuntary contractions are evoked that attempt to maintain balance and return the body to a position of equilibrium and this includes involuntary contractions of the hand muscles.

Enoka (2003) describes an example of an officer approaching a vehicle on the side of the road with his weapon in his hand. The officer suddenly slipped on a gravel slope and while falling experienced a contraction of the hand muscles causing his weapon to fire. Of course, the main contributing cause is the fact the officer violated proper procedure by having his finger inside the trigger guard, which permitted the contraction of the index finger to depress the trigger.

In order to further examine Enoka’s hypothesis, other researchers (Heim and Schmidtbleicher, 2006) conducted training where they asked officers to perform a variety of tasks while holding a firearm in their hand. The firearm was fitted with specially designed force sensors on the grip and the trigger so the researchers could determine precisely when and how much pressure was exerted. During one of the experiments, the researchers used a moving platform, which unexpectedly shifted and disrupted the participant’s balance.

In this study Heim found that when balance was suddenly disturbed, a significant number of the participants experienced sufficient involuntary contractile forces on the trigger to cause a weapon discharge.[1]

An example of loss of balance leading to unintended weapon discharge occurred in Houston, TX in June of 2016. A uniformed officer was attempting to split up a physical fight between four males in a crowded nightclub. As the officer was dealing with the situation he observed one of the males reaching into his waistband. Fearing that the male was attempting to access a weapon, the officer drew his handgun and as he did so, another bar patron suddenly pushed him off balance. As he lost his balance, he experienced an involuntary contraction and his gun discharged striking another, uninvolved, citizen in the bar.[2]

So much for losing our balance! In our remaining time this week, lets dive into another alarming cause of unintended discharge, the ‘Startle Response’

Incident: Officers had responded to an early morning intrusion alarm at a large middle school. Upon arrival, the audible alarm was no longer sounding (having been reset remotely by the monitoring alarm company). The officers did, in fact, find an insecure door to the school and decided to enter the school and ‘clear’ it to ensure no intruders were inside. They formed up a five-officer search team, drew their weapons, and entered the school through the insecure door. As the officers moved down a main hallway, they tripped the motion detector causing the audible alarm to go off. The unexpected sounding of the alarm resulted in the officer leading the search team, to have a sudden startle and since his finger was inside the trigger guard of his weapon, the officer’s gun discharged ripping a round down the (unoccupied) school hallway.

Unintended Firearm Discharge – Involuntary Muscle Contraction Caused by Startle Response

Another cause of involuntary muscle contraction is identified in the research as the ‘startle response’. The Oxford English dictionary defines ‘startle’ as ‘to cause to feel shock or alarm’. All people have, at one time or another, experienced the circumstance of being surprised or startled by a sudden and unexpected occurrence. When a startle response occurs, the body experiences a cascade of physiological responses that have been well documented in the research. Often also termed the ‘flight, fight or freeze’ response, one of the characteristic reactions of the human body under these conditions is rapid, involuntary and uncontrollable muscular contractions. In describing this phenomenon researchers Heim and Schmidtbleicher stated:

“Startle reaction, the third scenario identified by Enoka, is a whole-body reflex like response to an unexpected stimulus, usually a loud sound. It evokes rapid involuntary contractions that begin with an eye blink and spread to all muscles of the body (Landis and Hunt, 1939; Brown, 1995)”

Although most research has examined the startle response caused by an unexpected, loud auditory stimulus, it can also be evoked with visual or physical (somesthetic) stimuli.

Research has determined that the speed of this involuntary muscular reaction is extremely fast, occurring approximately 200ms after the presentation of the perceived threatening stimulus. Enoka (2003) concluded:

“Accordingly, an officer who is startled by a loud, unexpected noise while searching for a suspect with his weapon drawn would surely increase the grip force on the weapon, perhaps enough to cause an unintentional discharge”

The degree of involuntary contraction forces caused by the startle response is variable depending upon the context of each situation however, Davis (1984) found that the magnitude of the startle response and involuntary contractions increased when fear and stress arousal was present.[3]

In further research conducted by Heim (2006) exploring the startle response / involuntary contraction phenomenon, 66 officers volunteered to participate in a study. The officers were outfitted with specially designed weapons with force sensors on the grip and trigger and were sent into a realistic scenario-based environment where they were searching for an armed gunman who had just robbed a bank. Due to the high-risk nature of the event, the officers had their weapon in their hand as they moved through the scenario area but were directed before the initiation to follow their training and keep their trigger finger outside of the trigger guard. As the officers progressed through the scenario a subject would suddenly appear from a doorway approximately five meters from the officer and at the precise time this occurred, a researcher would suddenly fire a blank firearm directly behind the back of the officer. 6 of the 66 officers experienced involuntary contractions of significant force to actually fire their weapons, when they had made no decision to do so, and the speed of the contraction occurred an average 300ms after the startle stimulus.[4][5]

I am familiar with cases of unintended discharges occurring as a result of the startle response. In the incident case described above (which took place in Texas), the startle response led to an immediate and uncontrollable startle response causing the officer’s gun to discharge. Fortunately, nobody was injured in that incident.

A more striking, and far more tragic case of unintended discharge caused by the startle response occurred in New York City in 2004. At approximately 1:00 AM on January 24, 2004, NYPD officer Richard Neri Jr. and his partner were searching rooftops of a housing project in Brooklyn. As officer Neri approached a door, it was suddenly and unexpectedly opened from the other side by 19-year-old Timothy Stansbury who was walking along the rooftop with friends. The sudden opening of the door startled Officer Neri who had his Glock 19 service weapon in his hand. Neri testified before the grand jury that when the door opened suddenly, he was startled, and his gun fired accidentally.[6][7]

Well our time this week is up. I hope you are enjoying and being challenged by our exploration of the principle and contributory causes of unintended discharges in law enforcement. Please stop by next week for Part 5 in this series where we will explore uncontrollable contractions caused by hand confusion (brace yourself for ‘somatropic coordinates’!). We will also look at how ‘slip and capture errors’ have directly caused unintended discharges.

Until then – please take care and safe!


[1] Heim, Schmidtbleicher and Niebergall; “Towards an Understanding of Involuntary Firearms Discharges”; Policing: International Journal of Police Strategies and Management, Vol. 29 No. 3, 2006

[2] “Officer’s Gun Accidently Fired During Fight at Northeast Houston Nightclub” accessed August 27, 16 at http://abc13.com/news/hpd-officers-gun-accidently-fired-during-club-fight/1374662/

[3] M. Davis, “The Mammalian Startle Response”; In R.C. Eaton, Neural Mechanisms of Startle Behavior (pp. 287-351). New York: Plenum 1984

[4] Heim, Schmidtbleicher and Niebergall, “The Risk of Involuntary Firearms Discharge”; Human Factors: The Journal of the Human Factors and Ergonomics Society 2006, 48:413.

[5] Heim, Schmidtbleicher and Niebergall, “Involuntary Firearms Discharge – Does the Finger Obey the Brain?”; Firearms Instructor, Volume 43 2007.

[6] https://en.wikipedia.org/wiki/Shooting_of_Timothy_Stansbury

[7] http://www.nytimes.com/2004/02/18/nyregion/officer-avoids-indictment-in-killing-on-brooklyn-rooftop.html?_r=0