A packaged "incendiary device" exploded in a suburban St. Louis parking garage on Thursday, injuring the man who picked it up, rocking an office high-rise and forcing hundreds of people to evacuate.

The package was sitting near the 69-year-old man's assigned parking spot, but authorities wouldn't say whether he was the intended target. His injuries were not considered life-threatening, said Clayton Police Chief Thomas Byrne.

"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. 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.

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