The fatal shooting occurred at AK Home Health Care LLC, one several small businesses inside the Cherokee Place Business Incubator south of downtown St. Louis. The shooter gunned down another man and two women before turning his semi-automatic handgun on himself, Police Capt. Michael Sack said.
Authorities said the shooter either owned or was a co-owner of the small business and his three victims were employees.
"We don't know if this was a thing that carried over into today or was initiated today," Sack told reporters.
Police said surveillance video showed what appeared to be a verbal dispute, followed a short time later by gunshots penetrating an inside wall. The video showed that no one else had gone into the building other than the four people who were killed, the St. Louis Police Department said in details posted on its official Twitter account.
An employee of another business in the building heard gunshots and called police. Other businesses in the building include an attorney's office and an African bazaar.
The victims' names have not been released, but Sack said they appeared to be in their early-40s to mid-50s in age.
A woman who showed up about two hours after the shooting began sobbing loudly when she saw the police scene and was comforted by onlookers and police. A neighborhood woman translated the woman's outbursts for reporters, saying the woman was worried that a relative was inside the building.
Abdi Salam Elmi, an immigrant from Somalia who drives a cab in St. Louis, said he was close to all four of the dead in Thursday's shooting. He described them as hardworking, friendly people.
"They always smile for me. This is my worst day in my life. It's a very, very sad day for us and a very sad day for the city of St. Louis."
St. Louis has long struggled with urban violence, but the last week has seen a troublesome uptick in bloodshed. Police scrambled late Monday and early Tuesday to respond to five different shootings on the city's north side that left 15 people wounded.
Elmi said as a cab driver he sees too much violence in the city and he's concerned about the recent shootings. "I feel the same as I did when I left Somalia," he said, referring to the war-torn African country.
Meant to be a nurturer of startup businesses, the Cherokee Place Business Incubator dates back at least a decade in a once-thriving business section about a five minutes' drive from downtown.
Big retailers later shifted to the suburbs. But that part of town, which has a strong Latino flair, has regained solid footing. New street lighting complimenting welcomed police responsiveness has helped make it safe, according to Jason Deem, a board member and former president of the Cherokee Street Business Association.
Deem called Thursday's bloodshed "a very unfortunate situation for Cherokee" but not reflective of the area as a whole.
"It's not like this type of thing goes on down here. This is very much a shock to us," he said. "Everything police are telling us leads us to believe it was a targeted incident and not some random act of violence."