CHICAGO (CBS) —  An ambulance transporting a patient crashed into a building in Bellwood, killing three people.

The cause of the crash on Saturday, which occured at 28th Avenue and Washington Boulevard, remains under investigation. The Excel Ambulance Service driver, James Wesley, 51, a co-worker, Prentis Williams, 51, and the patient  all died.

The patient,  Larry Marshal Jr., 48, was being transported for dialysis.

“I can’t describe what that was like. It was the worst thing that has ever happened to me. My only son,” Minnie Marshall told CBS 2.

She said the family lost a dedicated brother, uncle and great uncle. Marshall recently underwent surgery and wasn’t able to drive himself.

So he relied on a private company to get to and from treatment safely.

“Just getting better from his illness and this happens. In an ambulance of all things,” Minnie Marshall said.

The ambulance company’s attorney, Frank Nagorka, said he believed this was the first crash for the company, adding Wesley was the business’ EMS coordinator for at least a decade.

“It’s a tragedy when such an event occurs, and we express our condolences to the family and all who knew these people,” says Nagorka.

Mayor Andre Harvey said it appeared the ambulance was making a left hand turn and lost control, crashing into a building.

Autopsies found no acute medical conditions caused or contributed to the death of the driver, according to a medical examiner’s spokeswoman.