Latvia has declared three days of mourning, beginning Saturday, for the victims of a deadly store collapse in the capital Riga.

Thursday evening's collapse killed 52 people and there are fears that the death toll might climb even further as people still remain trapped inside. Rescuers are also finding injured people who are being sent to hospitals, some in critical condition.

This is Latvia's deadliest disaster since it became independent in 1991.

Rescuers spent a second night sifting through the rubble.

They have been periodically turning off all their equipment and asking the families of missing people to phone their relatives so they can pinpoint the ring tones in the debris.

Growing numbers of people are arriving at the scene to lay flowers and light candles in commemoration of the dead, the BBC reports Saturday.

Three of those killed were emergency workers, who were helping people trapped, when more of the roof came down – 20 minutes after the initial collapse. The cause of the collapse is unclear although reports, quoted by the BBC, say a garden was being constructed on the roof at the time and the weight of the soil could be a contributing factor.

Rescue efforts continued through the night, involving several hundred firemen, police and emergency medical teams, and might last for another 24 hours.

Prime Minister Valdis Dombrovskis has announced that a criminal investigation in violating construction norms has begun.

The area has been sealed. The State Blood Transfusion Center has urged Latvians to donate blood for the injured.

The initial collapse happened just before 6 pm local time Thursday, when the store was busy with customers. In addition to the roof, walls and windows of the single-story concrete and glass building also crumbled.

TV footage showed rescue workers using mechanical cutters to clear debris from. Cranes were brought in to remove slabs of concrete. More than 60 soldiers are helping the rescue effort, the army, cited by The BBC, said on its official Twitter feed.

According to witnesses, customers tried to run out, but the supermarket's electronic doors closed, trapping them inside.

The supermarket, which opened in 2011, is part of the Maxima retail chain.