KOLKATA (TIP): At least 25 people were killed as the flyover in the Burrabazar area in North Kolkata, which was still under construction, collapsed on March 31.

Amidst the dust and debris, the big question was this: what led to the disaster? While a probe ordered by the government is expected to come up with answers, there were some clues on the ground.

While local residents confirmed that work was on through day and night over the last few months, an injured worker receiving treatment at the Marwari Relief Society hospital said that last night, his supervisors had found that rivets on the flanks of the flyover were damaged.

Said Partha Pratim Biswas, professor of construction engineering, Jadavpur University: “Wornout rivets or bolts could be a clear indication of the wear and tear, and rust, that the structure might have accumulated because of its long pendency.”

Officials said that the Left Front government had first come up with the idea of a flyover to decongest this choked Burrabazar stretch. Work began in 2007 but stopped two years later because of shortage of funds.

From the beginning, sources said, local residents were against the project because there was hardly a gap of a few feet between the flyover’s flanks and the balconies of some buildings on the side.

On Thursday, Army personnel engaged in rescue operations said they found that the iron sheets used as the base were “too thin and worn out”. “In some places, these iron sheets were twisted and did not appear to be of good quality,” said one of them.

Rajat Kumar Dutta, a government medical officer whose office is located just 50 metres from the accident site, said that he saw helmet-clad workers doing welding work on the flyover minutes before the collapse. “Within seconds, I saw the structure come down with those workers being tossed in the air,” he said. According to government officials, the project was barely 80 per cent complete by November. It was only the Chief Minister’s “intervention” that allowed work to start on a 150-metre stretch where work was held up for six years because of lack of permission from the Calcutta Port Trust to build piers for a ramp.