Police say the shooting happened shortly before 2 p.m.
As emergency vehicles arrived, dozens of employees poured out of the facility. One of them was Dandel Price's mother.
"I was shocked. I was worried. I tried to call my mom every five minutes to see if I could get an answer from her. And all of a sudden she finally called my brother. And that was a relief," said Price.
Price says he was told by his mother the male victim may have been a manager. Authorities say he was taken to St. Mary's hospital as the Kankakee County Sheriff's Department began looking for the alleged shooter, whom they identify as a plant employee.
Manteno resident Patrick Sweetie, who used to work at the plant and is now a Blue Island firefighter, received a first responder alert text about the incident.
"It's a quiet little town. We don't have crimes out here. We don't have unusual situations like this," he told ABC7.
Residents in a 55-and-older residential subdivision, a few hundred yards from the plant, were also alerted.
"An elderly gentleman came to the door and said that the police had told him that all the residents here in this neighborhood need to lock down and close the doors because there was a shooter loose," said Aimee Shugarman, whose relatives live near plant.
The Mars Chocolate company leases the facility, and a spokesman released a statement saying, "we are shocked by today's incident, and our thoughts and prayers go out to everyone involved. Mars Chocolate North America leases a portion of the warehouse through a longtime partnership. We will be supporting the management company, Four T's Management, along with the police throughout the investigation in any way that we can."
Mars did say that neither the victim nor the alleged shooter is a Mars employees.
Manteno is about 45 miles south of Chicago.