A bridge under construction over the Kalnagini river in Kakdwip, South 24-Parganas, collapsed on Monday morning.

The Sunderbans Affairs department started building the 120m-long bridge two years ago and almost 50 per cent of the work was over, an official in Nabanna said.

The bridge was meant to connect Steamerghat and Paschim Gangadharpur.

“The local administration did not report any death but we are not taking the matter lightly,” the official said. “We are trying to find the actual reason for the collapse.”

Government engineers who visited the spot said primarily it appeared the executing agency was inexperienced.

“The girders were placed over the piers a few days ago and the deck slabs were still to be placed over them. It is unusual for the girders to collapse under the weight of the deck slabs,” an engineer said.

“It appears that the steel support to the girders on the ground was removed before they settled on the piers. Only inexperience on part of the

executing agency can lead to such a situation.”

The standard of material used and faulty design can’t be ruled out, though, the engineer said.

The state government will investigate the matter, Partha Chatterjee — the education minister who is the chairman of the group of ministers formed by the chief minister to look into important matters in her absence — said.

“The bridge was under construction… We are trying to the find out the reasons... we will investigate if the quality of materials was substandard or there was technical fault,” he said. “Inexperience of the executing agency is being considered as well.”

A group of children had a narrow escape as they were playing a few metres off the site of collapse.

Residents alleged that the material used in construction was of poor quality. They claimed to have told engineers visiting the site during construction about cracks in the structure.

“We alerted them about the cracks. But they did some patchwork and covered the cracks,” a resident said.