RIGA, Latvia (AP) — Large sections of roof collapsed Thursday at a grocery store in Latvia’s capital, killing six and injuring at least 30, rescue officials said. Firefighters and soldiers were searching for survivors amid the debris.

The collapse took down the Maxima supermarket’s high walls and windows in a ripple effect that filled the shell of the building with rubble and left shattered glass on the street. The cause was still unknown, but workers were reportedly planting a winter garden on the store’s expansive grass-covered roof.

Two firefighters were among those killed, and seven were injured after another portion of roof fell on rescue workers as they began searching for survivors in the debris, according to the Riga Fire and Rescue Service.

Ilze Buksa, a spokeswoman for Latvia’s emergency medical service, said 22 people were being treated in hospitals with various injuries, some of them serious. Many injured received treatment on site and went home.

It was unclear how many people might be trapped, but the store is in a densely populated part of the capital not far from the city center. Authorities say the accident happened at about 6 p.m. local time, during the after-work rush hour.

Viktorija Sembele, a spokeswoman for the Fire and Rescue Service, said the cause of the collapse, which included approximately 500 square meters (5,300 square feet) of roof, was not clear. Witnesses have told Latvian media they heard an explosion, but there were reports that a garden with trees and plants was being built on the store’s sizeable roof.

Sembele confirmed that there may have been additional construction, which apparently included a large amount of soil and dirt.

The grocery store is part of a 5,000 square meter (53,000 square feet) shopping center that was completed in November 2011, and incorporated a large, grassy area on the roof.