[10][11] At 11:18 p.m., a gunman, later identified as Ian David Long, entered the Borderline Bar and Grill and opened fire on the approximately 260 patrons and employees inside.

[5] Long was armed with a legally purchased .45-caliber Glock 21 semi-automatic pistol with a Viridian X5L tactical weapon light and laser sight[8] and seven banned high-capacity magazines,[17] carrying a total of 190 rounds, along with a folding knife, ten smoke bombs, and two fireworks.

[3][21] At 11:19 p.m., two California Highway Patrol (CHP) officers on a traffic stop nearby were alerted to the shooting by people who had managed to escape.

[12][22] In the ensuing gunfight, Helus was shot five times by Long, who used a flashlight with a laser sight on his pistol in the large, darkened, smoke-filled room.

Responding Ventura County Sheriff's deputies, who were securing the perimeter, located and evacuated Helus outside the building 20 minutes after he had been shot.

[12] He served in the United States Marine Corps from August 2008 to March 2013, reaching the rank of corporal, and he had gone to Afghanistan from November 2010 to June 2011.

After being honorably discharged, Long attended California State University, Northridge as an athletic training major from 2013 to 2016, but he did not graduate.

[12] Police and a mental health crisis team visited Long in 2017 for his irate and irrational behavior, but they decided not to detain him at a psychiatric facility.

[40][41] A high school teacher raised claims that Long had physically assaulted her as a student but she was encouraged not to push the incident so as to not endanger his future in the Marine Corps.

[42] Although he had served overseas in the military, behavioral scientist and clinical psychologist Lisa Jaycox said that it was premature to say whether Long suffered from PTSD or if it was a factor in the shooting.

[38][11][43] A 434-page report from the Ventura County Sheriff's Office was released in June 2021, which detailed the findings of the investigation into Long's actions and motivation.

However, the Ventura sheriff stated that the investigation did not show with absolute certainty that a hatred of college students was the motive, instead it was more a working theory.

The report had been compiled with information from interviews with eyewitnesses, officers and deputies, and family, friends and acquaintances of Long and was a joint effort by Ventura County sheriff's detectives and FBI analysts.

Initially composed of key county officials, the ongoing task force looks for gaps in the systems that protect the public and that provide mental health care and makes recommendations on how to prevent and lessen the damage from mass shootings.

[55] In 2022 the Ventura County Sheriff's Office released to its website multiple audio and video recordings from the shooting after being sued by The Star and other media organizations for access to the materials.

In total the county released all recordings of police radio communication, ninety-four emergency calls, security footage from inside the bar and fourteen dashboard and body-camera videos from responding officers.