The suspect, who Action News learned has made threats in the past, is in custody. That person's name has not been released.
The shooting happened around 8:30 a.m. at Delaware County Linen in the 2600 block of W. 4th Street.
Officials confirmed the people who died were employees.
"Two individuals who, unfortunately, have lost their life just by showing up at work today," Delaware County District Attorney Jack Stollsteimer said.
It's not yet known what motivated the suspect to open fire, and whether the victims were specifically targeted.
Employees say they know the suspect well. One woman told us he was an ongoing problem.
"A lot of people put a complaint for him in the office," the worker said. "No listen."
That worker was on vacation Wednesday but she said she came to the scene when she heard what happened.
She told Action News the gunman was rather aggressive and often wore a gun on his belt. She told us the man said he didn't care and would be fine shooting people.
Employees said, at first, they thought the sound of gunshots were actually an explosion, possibly from the chemicals used in the cleaning process.
That was until they heard co-workers telling them to get out of the building and take cover.
"We're all scared because of what happened. We saw somebody was coming, but we didn't stay to see what would happen. Everyone just ran for their lives," said another employee.
One of the people who was shot is a supervisor at the business, Chester Mayor Stefan Roots said. It's not known if that supervisor was among the dead.
Eric Spencer said he was a witness to the shooting. He says it started as an argument between several of his coworkers, including the gunman.
"I was standing there for a minute but then I walked away because I don't understand Spanish. Next thing you know, I turned around, he pulled a gun out of his pocket and point-blank range, just like that," he said.
Spencer said he heard the rest of the gunshots but didn't see them because he was already running away - and for good reason.
"We just argued yesterday," he said, referring to the gunman. "My friend grabbed me, he said, 'Come on, let's go, let's go.'"
Chopper 6 over the scene after shooting at business in Chester, Pa.
After the shooting, the suspect fled the scene in his personal vehicle.
However, he was stopped by a police officer from Trainer, Pa., who heard the vehicle description. The suspect was taken into custody at Kane and Culhane streets in Chester.
A police source and a neighbor say that's where the suspect lives, so it appears he was driving home after the shooting.
The neighbor said she saw the suspect on the ground being arrested. She added that the family recently moved to the area from Puerto Rico.
"I looked out the window after the police sirens woke me up," said neighbor Christina Martinez. "And there's my neighbor, on the ground, getting arrested and they took him away. It was like a movie."
Other neighbors said they are a nice family and they're shocked about Wednesday's events.
A gun was found inside the suspect's car, officials say. It wasn't immediately known if that was the weapon used in the shooting.
The three victims who were wounded are said to have serious injuries. One of the injured victims was rushed to the hospital by Chester police officers.
The victims' names have not been released.
"It speaks to guns in America," said Roots, who said his small city has enjoyed a period of relative calm in recent years, with violent crime on the decline. "Violence is always unpredictable. We don't know what conditions people are under, in a city that's impoverished like ours."
Roots called on state lawmakers in Pennsylvania to address the gun crisis, especially as it relates to people with mental health problems.
"We can't have guns in the hands of the wrong people," Roots said. "There had to be some type of mental health issue that would have an employee walk into his workplace on a day of work and take out his anger in such a violent way - to not just the boss, but his coworkers, who he probably worked side-by-side with for years."