"He picked up a package sitting next to his car and it exploded," Byrne said, calling the package an "incendiary device" but not elaborating.
"We don't know who set it or why it was there," Byrne said.
The parking garage is shared by office and residential buildings. No damage was apparent from outside the complex.
A Ritz-Carlton Hotel also sits nearby in Clayton, a busy, well-to-do suburb that is the seat of St. Louis County and home to many of the region's biggest law firms, financial offices and other white-collar businesses, as well as posh hotels and restaurants.
The explosion shortly after 11 a.m. rocked the high-rise building, witnesses said. Buildings were evacuated, leaving several hundred people to mingle for hours on a lawn.
By mid-afternoon, police and a bomb-sniffing dog were still searching the office building for any additional devices, but people were allowed to return to the residential building and the hotel.
Lisa Pogue, 51, secretary for a law firm in the office building, said she heard a boom and felt the building shake. Fire alarms went off, prompting a mass exodus.
"It was alarming," she said. "There was definitely something wrong."
The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives were helping local authorities investigate, Byrne said.
Slideshow archive | ABC13 wireless | Help solve crimes