RIGA, Latvia—The death toll in Thursday's building collapse near Latvia's capital city rose to at least 51 people, in one of the worst tragedies to hit the small Baltic nation in decades.

The roof of a supermarket operated by the Lithuanian-owned Maxima retail chain collapsed around 6 p.m. local time Thursday, trapping dozens of shoppers and employees inside. Another chunk of the roof fell after rescue workers arrived, killing three firefighters.

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