RIGA, Latvia — At least 26 people died, including three firefighters, after large sections of roof collapsed at a Latvian, the country's rescue service said Friday.

The reason for the collapse on Thursday was still not known but rescue and police officials said that a winter garden, involving large amounts of soil, was being built on the roof.

Interior Minister Rihards Kozlovskis ruled out natural disaster or terrorist activity as a possible cause and said that in all likelihood construction regulations had been violated and triggered the collapse.

The Fire and Rescue Service said 35 people were injured, 28 of whom are currently hospitalized, including 10 firefighters.

The rescue service estimates that approximately 500 square meters (5,300 square feet) of roof collapsed, destroying large sections of the store's high walls and nearly all its front windows.

A large contingent of rescue workers, including soldiers and dogs, continued searching for survivors Friday morning at the store in a densely populated neighborhood. Four large cranes hovered over the supermarket as they tried to lift large pieces of construction material.

Rescue workers periodically turn off all equipment and ask relatives of missing people to call so that they can pinpoint the ringing phones, rescue service spokeswoman Viktorija Sembele said.

She said work was proceeding slowly since both the rubble and remaining sections of roof were like a house of cards and could easily collapse further if the wrong piece is moved or lifted.

Firefighters suffered casualties when large sections of roof fell on them right after their arrival on the scene.

Rescue officials are unsure how many survivors may be under rubble, but they expect the death toll will rise.