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Behavioral Threat Assessment of Samuel James Cassidy

POI: Samuel James Cassidy

Kayla Barnes, Vivien Popper, Angeliki Siafaka, Behavior/Leadership (B/L) Team


There remains limited information available about the personal history of Samuel James Cassidy, and following this devastating incident, many concerned parties wanted to know if this attack was a small piece of a larger domestic terrorist plan. After conducting an analysis on the perpetrator, Samuel Cassidy, we have reached the following assessments:

  • Based solely on current information that is publically available, it is not yet appropriate to classify Cassidy as a terrorist, nor the attack as an act of terrorism.

  • Cassidy acted alone with the likely motive of harming coworkers that he had a grievance with.

  • Cassidy had run-ins with law enforcement and made concerning comments that, if reported to VTA, may have resulted in an intervention.

It is unlikely that a future threat will be derived from this particular event. However, it is important to continue monitoring the situation for updates and performing research to find and dissect the motive to prevent future copycat threats. This analysis would be able to occur more successfully with more information about Cassidy’s personal history.

The VTA (left) and Samuel James Cassidy (right)[1]

Event Overview

In San Jose at 0634 local time on Wednesday, May 26, 2021, Samuel James Cassidy, 57, arrived at the Valley Transportation Authority (VTA) in a Ford F-150 pickup. Cassidy carried three semi-automatic handguns and 32 high-volume magazines. He entered the rail yard heading to Building B on the western side of the area and started the gunfire. Afterward, he went East to Building A. Soon after Cassidy commenced the shooting on the premises, Santa Clara County Police received 911 calls, and the deputies arrived at the scene, going in from North San Pedro Street. Meanwhile, at 0636 local time, firefighters were called to an incident at 1100 block of Angmar Court in San Jose. Due to the extent of the damage by the fire, the investigation of the home could only begin an hour later. It was revealed that Cassidy owned the house. Santa Clara County deputies encountered Cassidy in Building A, where he took his own life. Samuel James Cassidy worked at the VTA and targeted his coworkers, killing nine people in the mass shooting. After investigating Cassidy’s home, authorities found more ammunition and weapons.

Building A at the VTA, where some of the victims were shot[2]

POI Overview - Items of Interest

According to early media coverage of law enforcement commentary, Cassidy was described by coworkers as observably disgruntled before the attack. Cassidy was stopped by the United States (U.S.) Department of Homeland Security (DHS) Border and Customs Protection after a flight back to the U.S. from the Philippines, during which authorities discovered “books about terrorism and fear and manifestos…as well as a black memo book filled with lots of notes about how he hates the VTA [Santa Clara Valley Transportation Authority],” though when prompted, Cassidy reported that he did not have any issues with coworkers.[3]

Observable Indicators and Characteristics

To indicate whether this event was a lone-actor terrorist attack, we have assessed Cassidy using the Terrorist Radicalization Assessment Protocol (TRAP-18), a structured professional judgment instrument for those engaged in risk assessment of persons of concern for acts of terrorist violence. The TRAP-18 consists of 8 proximal warning behaviors and 10 distal characteristics which have been displayed throughout past terrorist attacks (for more information about the scientific method, validity, current usage of the TRAP-18, and definitions of the behaviors and characteristics, see the provided source).[4]

Only the evidence which has been proven and confirmed are checked on this list. Unchecked boxes may or may not have been exhibited by the POI but are, for now, unknown. While many theories exist, a motive has not yet been finalized by the FBI.

Classification as Terrorism

According to the creators of the TRAP-18, the majority of lone actors were positive on 13 of 18 TRAP-18 variables (72%).[5] These individuals showed a frequency of >70% on the distal factors of personal grievance/moral outrage, framed by an ideology, thwarting of occupational goals, and changes in thinking and emotion, and 77% or more on the four proximal warning behaviors, namely pathway, fixation, identification, and leakage.[6] This means that lone-actor terrorists tend to display a pattern of common pre-attack behaviors that distinguish them from other criminals. While this is not a fool-proof method, using the TRAP-18, we can compare Samuel Cassidy’s traits to those of known lone-actor terrorists to help us determine whether this attack was a terrorist act or simply motivated by a personal grievance.

Based on the publicly available information, Samuel Cassidy displayed 11 total indicators that align with other lone-actor terrorists. Therefore, there are similarities between Cassidy and the behaviors displayed by terrorists, and as more information reaches the public, we may be able to characterize him as such. While it is easy to assume that this was an act of domestic terrorism due to the mass casualties, we assess that there is not enough evidence to label this as an act of terrorism. An act of terrorism aims to send a broader message or serve political objectives, but from analyzing the available information, it seems that Cassidy’s attack was perpetrated for personal reasons and due to his anger towards his co-workers. We assess that even though the goal was a mass casualty, the shooting was motivated by Cassidy’s personal grievances and what he perceived to be unfair treatment at his workplace.

Target and Goal

The target of the attack was the Santa Clara Valley Transportation Authority (VTA), and it is highly likely that Samuel Cassidy targeted his workplace and selected his victims driven by his personal grievances. Samuel Cassidy had worked for Valley Transportation Authority since at least 2012, first as a mechanic from 2012 to 2014 and then as a substation maintainer.[7] Law enforcement agencies have confirmed that Cassidy had a long history of dissatisfaction with his employer.[8] The Santa Clara Sheriff’s Department said that Cassidy was a highly disgruntled VTA employee for many years, contributing to why he targeted VTA employees.[9] Cecilia Nelms, Cassidy’s ex-wife, stated that he was always angry about things that happened at the VTA and he would tell her that he wanted to kill people at work.[10] Therefore, the personal connection and the history of workplace conflict lead us to conclude that personal grievance led Cassidy to target that location and that he likely operated alone as opposed to alongside a group and/or other co-conspirators. Additionally, Cassidy knew all of his victims very well.[11] A witness who survived the shooting said that Cassidy did not fire indiscriminately, but instead, he let some people live and chose to kill others.[12] Another witness said that Cassidy spoke directly to a few employees during the attack, telling them that he was not going to shoot them before turning to shoot other people.[13] This suggests that Cassidy bypassed certain people and carefully selected those that he wanted to kill. Surveillance footage shows Cassidy walking through the rail yard as he moved between different buildings of the VTA.[14] Cassidy likely went into different buildings of the VTA as he was looking for the people he wanted to shoot. We assess with a high level of confidence that the target of the attack was the Santa Clara Valley Transportation Authority (VTA) site in San Jose, California, and that Samuel Cassidy acted alone. We assess that Cassidy specifically targeted his workplace and that he selected his victims due to personal grievances.

According to the Santa Clara Sheriff’s Office, this attack was a planned event and Samuel Cassidy was prepared to use his firearms to take as many lives as possible as he had collected 12 firearms, at least 20 cans of petrol, and approximately 22,000 rounds of ammunition.[15] The shooting happened during the busiest time of the day at the VTA site when operators and maintenance workers were getting ready for the start of the day’s service.[16] In addition to the three semi-automatic handguns and the 32 loaded high-capacity magazines he was carrying when he opened fire, Cassidy had stored materials for bombs, detonator cords, and the precursors to an explosive in his locker at the rail yard.[17] Cassidy had also read up on terrorism, manifestos, and harbored bomb-making materials.[18] Thus, it seems likely that Cassidy was very well prepared for a deadly attack aiming at mass casualty. However, with Cassidy ultimately choosing to use firearms, it is probable he did not opt for explosives or bombs because his goal was not indiscriminate killing but rather revenge on the co-workers who, in his eyes, had wronged him. We assess that mass casualty was the goal of the attack and, considering that Cassidy seems to have selected his victims, we assess that Cassidy aimed to express his anger and resentment towards his co-workers for being mistreated.


An official motive of Samuel James Cassidy’s mass shooting has not been confirmed yet. Based on the available information, the most likely influencing factor was his hatred towards the Valley Transportation Authority (VTA). In 2016, when he returned from the Philippines, U.S. Border Control apprehended him and found materials about terrorism, manifestos, and notes on contempt against the VTA.[19] This suggests that Cassidy had been preparing violent plans directed at his workplace for a long time. Something likely happened in the previous days, which set the violent behavior off, resulting in the shooting. According to his ex-wife, Cassidy had talked to her prior about killing his colleagues at the VTA. These workplace threats also support the hypothesis that his motive was strongly related to personal problems at VTA and specific colleagues. Additionally, Cassidy’s family members and acquaintances said he had alcohol problems and suspected mental health issues, namely paranoia and bipolar disorder. As Cassidy has never received a diagnosis, it is unclear to what extent these illnesses influenced the event. Cassidy was often described as an outsider and isolated individual. Besides his impulsive temper and anger issues, he had no real personal connections; the failed marriage and poor relationships can be strong indicators of personality disorders.[20] Taking his own life also shows his determination to carry out his plan with no limitation and indicates that he was likely prepared to kill himself or be killed by the authorities in the end. As no real motive has been confirmed yet, these factors seem to be the most viable in explaining the intention behind Samuel James Cassidy’s actions.

Conspiracy Theories and Copycats

There are no recorded conspiracy theories believed by the offender, nor have there been any documented copycats.

Future Implications

This case shows the significance of leakage and indicators toward the pathway to violence, illustrating how law enforcement should have a path to communicate troubling information to an individual’s employer if necessary. Cassidy had displayed troubling sentiment toward VTA for years before the attack, as recounted by his ex-wife. Though this information is not particularly troubling in isolation, DHS’s discovery of a manifesto, terrorism literature, and documents recounting hate toward VTA should have been promptly communicated to VTA so that the organization could launch its own investigation. However, the failure of law enforcement to communicate this information allowed an insider risk to develop into a fully actualized threat. Moreover, Cassidy’s position as a public transportation worker, a part of the U.S.’ critical infrastructure, is an additional reason to take prompt action on credible threats. Disruptions to transportation can result in exacerbated injury, halted supply chains, and harm to the overall lives of citizens. Local, state, and federal law enforcement could take this incident as an opportunity to explore better communication pathways between law enforcement and private and public sector businesses when an individual is being investigated for actions that may implicate future violence while remaining in accordance with any privacy laws.


[1] San Jose gunman Samuel Cassidy faced disciplinary hearing on day of attack, Twitter, May 2021,

[3] San Jose mass shooting: VTA shooter’s family apologizes, says he felt lost to them for months, Mercury News, May 2021,

[4] “Assessing the threat of lone-actor terrorism: the reliability and validity of the TRAP-18,” Forensische Psychiatrie, Psychologie, Kriminologie, 2020,

[5] “Assessing the threat of lone-actor terrorism: the reliability and validity of the TRAP-18,” Forensische Psychiatrie, Psychologie, Kriminologie, 2020,

[6] Ibid.

[7] Authorities ID 8 victims of California railyard shooting, The Associated Press, May 2021,

[8] Surveillance video shows VTA gunman walking between buildings during mass shooting, ABC7 News, May 2021,

[9] San Jose gunman had 22,000 rounds of ammunition at his house, which sheriff's office thinks he set on fire, CNN, May 2020,

[10] Authorities ID 8 victims of California railyard shooting, The Associated Press, May 2021,

[11] Gunman who killed eight co-workers at California transit facility knew victims well, mayor says, CNN, May 2021,

[12] Surveillance video shows VTA gunman walking between buildings during mass shooting, ABC7 News, May 2021,

[13] 'Such extreme steps': San Jose killer intricately planned rail attack with a simple motive in mind, USA Today News, May 2021,

[14] San Jose shooting: VTA gunman was ‘highly disgruntled,’ had 32 illegal high-capacity magazines, East Bay Times, May 2021,

[15] San Jose shooting: Guns, petrol and 22,000 rounds of ammunition found, BBC, May 2021,

[16] Victims, shooter identified in Bay Area’s deadliest mass shooting, Mercury News, May 2021,

[17] 12 guns, bombs and 25,000 bullets recovered from home of VTA shooter, Mercury News, May 2021,

[18] 'Such extreme steps': San Jose killer intricately planned rail attack with a simple motive in mind, USA Today News, May 2021,

[19] San Jose mass shooting: VTA shooter’s family apologizes, says he felt lost to them for months, Mercury News, May 2021,

[20] Symptoms - Borderline personality disorder, NHS, July 2019,



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