Timothy Gerard Hendron was a 51-year-old employee at the ABB transformer plant in St. Louis, Missouri. He had worked at the facility for over two decades, initially as a supervisor before being reassigned to a line worker position. Colleagues noted that Hendron had become increasingly disgruntled, particularly after a demotion and involvement in a class-action lawsuit against ABB concerning the company's pension plan. The lawsuit alleged that ABB's 401(k) plan included investment options with "unreasonable and excessive" fees. Hendron was reportedly anxious about testifying in the case, which was set to go to trial shortly after the shooting.

On the morning of January 7, 2010, around 6:30 a.m., Hendron arrived at the ABB plant armed with multiple firearms, including a Romarm Cugir 7.62×39mm semi-automatic rifle, two Hi-Point semi-automatic pistols (.40 S&W and .45 ACP), a Tri Star 12-gauge pump-action shotgun, and a third handgun. He carried hundreds of rounds of ammunition.

Hendron began his attack in the parking lot, fatally shooting two employees: Carlton Carter, 57, and Terry Mabry, 55. He then entered the facility, where he killed Cory Wilson, 27, his supervisor, who was hiding in a storage area. Five other employees were injured during the rampage, two of whom were critically wounded.

During the chaos, one employee retrieved a handgun from his vehicle and attempted to confront Hendron but was shot and critically injured. Hendron fired approximately 115 rounds before turning the gun on himself, dying from a self-inflicted gunshot wound.