"The parts of the roof that collapsed were covered with a layer of frozen snow that was 50 centimetres (1.6 feet) deep," Transport and Buildings Minister Jerzy Polaczek told a press conference in Chorzow, making public the preliminary findings of a probe into the disaster.

But Grzegorz Slyszyk, a spokesman for the company that managed the building, insisted accumulated snow had not caused the disaster.

"That has been ruled out. All the documentation about snow removal has been handed over to the prosecutor's office in Katowice," Slyszyk told a press conference.

"The roof was checked and cleared of snow on a regular basis," he had said earlier.

The exhibition hall, where at least 200 people were attending a racing pigeon show when the roof collapsed on Saturday, met all Polish safety regulations, said Slyszyk.

But at the press conference, he pointed to possible structural faults in the building, which dated from 2000, as a cause of the accident.

"We must not overlook the hypotheses that mechanical resonance could have caused the disaster, given the noise inside the building, or the difference in temperatures outside the hall, where it was minus 15 degrees Celsius, and inside, where it was 20 degrees Celsius," said Slyszyk.

Land subsidence, which is frequent in Silesia, a mining region, could also have been behind the tragedy, he said.

Despite a massive rescue operation that ran through the night in icy temperatures that fell to minus 17 degrees C (one degree Fahrenheit), 66 people died and 141 were injured in what Polish President Lech Kaczynski has called the "biggest catastrophe in democratic Poland".

Following the accident, Kaczynski cancelled an official visit to the Czech Republic, due to begin Monday, and Prime Minister Kazimierz Marcinkiewicz called off a two-day visit to Sweden, also set to start Monday.

Marcinkiewicz said a special commission to investigate the causes of the accident would begin work on Sunday.

Polish Interior Minister Ludwik Dorn ordered Sunday that the roofs of all public buildings be cleared of snow.

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