Story highlights The death toll from a supermarket roof collapse in Latvia Thursday has risen to 53

Latvia's president has described the collapse as "basically mass murder" of defenseless citizens

President Andris Berzins has called for foreign independent experts to investigate

Russia's state media is reporting that two of its citizens are among those killed in Riga

Latvia's president has described the deadly collapse of a supermarket roof in Riga as "basically mass murder" of defenseless citizens, local media reports.

At least 53 people died in Thursday's collapse at the Maxima supermarket, in the west of Latvia's capital, national news agency LETA said Saturday . It said rescue workers are continuing their hunt for survivors and still have about 150 squre meters to search.

President Andris Berzins had told Latvian television that "nature could not be blamed" for the collapse and that those involved needed to think about what they had done, so that action could be taken.

"This incident is basically the mass murder of a large number of defenseless civilians. We must act accordingly,'' Berzins said.

Berzins urged for a team of independent experts from abroad to be assembled to investigate the collapse. Latvia has declared three days mourning -- Saturday to Monday -- for victims of the collapse.

It said a second collapse had occurred after rescue workers arrived and a third took place Saturday, but that no rescuers were injured in the latest collapse.

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"The area is very dangerous and unstable, and the decision has been made to search the area with less men, so to prevent anymore collapses," the news agency said.

Three firefighters are among the dead. Latvia's Interior Ministry said their families would receive 50,000 LVL ($95,600) in compensation. Treatment for rescue workers injured in the operation would also be paid for, it said.

In an interview with CNN Friday, Rescue Service spokeswoman Viktorija Sembele declined to speculate on the cause of the collapse.

"There are a lot of versions, a lot of stories. The real cause of this tragic accident will be investigated by state police and other authorities once the rescue is finished," she said.

However, Riga Mayor Nils Usakovs earlier told CNN that authorities think building materials stored on the roof caused it to collapse.

The supermarket collapse is the Baltic state's deadliest accident since it won independence from the Soviet Union in 1991, LETA said. Latvia's previous most deadly accident was a nursing home fire that killed 26 people in 2007.

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