A 15-year-old girl who was swimming with her mother at the Bidhannagar Municipal Sports Academy swimming pool, opposite Mayukh Bhavan in Salt Lake, died after she reportedly passed out in the water on Tuesday morning, police said.

Officers said on Tuesday evening the exact cause of her death was yet to be ascertained.

Elina Dutta Bhattacharyaa, 15, a Class X student at New Town School, had learnt swimming at least five years ago and was a regular, along with her mother Dalia, at the swimming pool run by the Bidhannagar Municipal Corporation, a family member said.

The 10-lane pool is 50m long and 25m wide. The water is 8.5ft deep at the deep end and 3.5ft deep at the shallow end, said an official of the civic body.

On Tuesday, Dalia, who was in the same lane as Elina, noticed that her daughter did not come up for breath after she dived into the deep end of the pool.

“Elina was training to increase her endurance and would often swim underwater for some time before she came up to breathe and dive again. On Tuesday, too, she was doing the same,” the family member said, requesting anonymity.

Dalia, who was familiar with this exercise, thought her daughter would come up for breathe. When Elina did not, she raised an alarm.

“Two trainers immediately dived underwater near the place where Elina was seen. She was then brought out of the water and administered cardiopulmonary resuscitation (CPR) by a trainer who is trained in the technique,” a senior official of the Bidhannagar Municipal Corporation said.

She was then taken to a private hospital near City Centre, where doctors declared her dead on arrival.

An officer in the Bidhannagar Police Commissionerate said a case of unnatural death has been registered. The body has been sent for autopsy.

“We have started a case of unnatural death and a probe,” said Badana Varun Chandrasekhar, joint commissioner, headquarters, Bidhannagar commissionerate. There was no complaint from the family till Tuesday night.”

Tapas Sengupta, secretary of AE Block Samaj Kalyan Sangha, the residents’ association of the AE (Part II) block, said Elina used to take part in cultural programmes in the block every year.

“Elina was a good swimmer and was quite jovial. She used to take part in swimming competitions and cultural programmes in our block regularly,” said Sengupta.

The swimming pool does not have a lifeguard, said a senior official of the Bidhannagar Municipal Corporation.

Mayor Krishna Chakraborty said the pool will remain shut till the civic authorities conduct a thorough review of the infrastructure.

“We are planning to review the infrastructure and will hire lifeguards. We had an adequate number of trainers but we did not have lifeguards. We will hire them on an emergency basis and the pool will only open after that,” said Chakraborty.

A senior official of the civic body said they were overwhelmed by the number of swimmers enrolling.

“More than 2,500 members enrolled this year. In each batch we have 40-50 swimmers and non-swimmers (learners). The sheer numbers are proving to be a challenge for the trainers. We need more trainers,” said the official.

There were around 35 trainees and swimmers in the pool when Elina passed out in the water, the official said.

Members have to pay Rs 2,000 for every season and Rs 500 for an application kit. Each member is allotted a 45-minute time slot.