Personnel and heavy equipment deployed for the search and rescue operation at the site of the collapsed under-construction warehouse in Garden Reach were withdrawn on Sunday, more than four days after the accident that claimed 16 lives and left 17 others injured.

The rescue operation was officially called off at 4pm on Saturday, bringing to an end a more than 74-hour effort that began soon after the warehouse collapsed on Wednesday.

Rescuers pulled 33 people from beneath the debris of the roughly 6,000sqm structure, which is suspected to have collapsed because of faulty engineering. Seventeen of those rescued survived.

The National Disaster Response Force (NDRF), which played a key role in the operation, dug nearly 50 “rat holes” through the debris to reach people trapped underneath, whether alive or dead.

At least two rescue personnel said some of what they witnessed inside the rubble defied scientific explanation.

“In most buildings, roof casting is carried out floor by floor, from the ground upwards. In this structure, we found that the roof slabs on the second and third floors had been cast, but the first-floor slab had not. It appeared the work was being done in reverse order,” said a member of the rescue team.

Officials from both the National Disaster Response Force (NDRF) and the police said they had found several apparent deficiencies in the design and construction of the warehouse.

“I saw that the galvanised iron-sheet roof had been covered with cement. That defies all engineering logic. The cement layer unnecessarily increased the weight of the roof,” an NDRF rescuer said.

During the operation, the NDRF dug around 50 rat holes through the debris and inserted victim locator cameras to detect signs of life.

“We have checked every inch of the site. The victim locator cameras were inserted through the rat holes to scan the debris. We believe no one else is trapped underneath. There could, however, be an exception if someone is caught in an inaccessible air pocket,” the rescuer said.

Of the 33 people pulled from the debris during the three-day rescue operation, 16 were declared dead. Of the 17 injured, 13 have since been treated and discharged.

Police have so far arrested six people in connection with the collapse.

On Sunday, officers of the special investigation team probing the incident searched the Howrah residence of Kalicharan Banerjee, a former officer on special duty to former Calcutta mayor Firhad Hakim, and seized documents and other materials.