The incident happened Friday morning at the McBride Lumber Company Warehouse in Star. A family-owned business, the company makes pallets and treated wood for furniture and other products.
Montgomery County Sheriff Dempsey Owens said 50-year-old Ronald Dean Davis, who worked at the lumber warehouse, apparently believed his Hispanic co-workers were mocking him when they spoke in Spanish and laughed.
"It appears he went into the building with a loaded 12-gauge shotgun, went up to these four individuals and shot each one of the four," Owens said.
The sheriff identified the victims as 34-year-old Eusebio Diaz Gomez of Asheboro, Davis' cousin, 32-year-old Daniel Thomas Davis and 25-year-old Jose Galdino Lopez Izquierdo of Asheboro.
Twenty-one-year-old Florentino Tellez Aparicio of Candor is in fair condition at UNC Hospitals in Chapel Hill.
About 16 other employees were at the McBride Lumber Co. when Davis started firing.
After the shooting Friday, Davis was found at his home with a gunshot wound.
"One of my deputies observed Mr. Davis sitting with his head on his chest," Owens said. "Our deputies entered the residence and found there was a single gunshot wound to Mr. Davis and he was still breathing at that time."
Davis was airlifted to UNC Hospitals in Chapel Hill, where he died Saturday morning.
At his home, deputies found a six-page handwritten note saying he thought co-workers were punishing or picking at him.
"The note did mention some problems he was having with some employees," Owens said. "He felt that certain people would be in a group and laughing, and he'd turn around and felt like they were laughing at him due to the language barrier between English and Spanish."
Workers and the lumber company factory owner told authorities they knew nothing about that, the sheriff added.
The note also mentioned medical issues Davis said he had, but the sheriff said there was no obvious reason given for what he did.
"I can't explain why or why not," he said. "He's not here to interview and try to learn from this."
Autopsies have revealed that all the victims were shot twice - the second time at close range after they fell.