Nine days after the major mishap at state run-NEEPCO's Kopili hydel plant in Assam's Dima Hasao district where four workers are trapped since October 7 following flooding of the power station, rescuers recovered a pair of slippers, a bag and a cap on Tuesday, police said.

"The bag and the blue cap belong to my brother," said Love John, whose 38-year-old brother Robert John, a junior engineer with NEEPCO is among the four who likely rushed to the last floor of the power station, three levels below the ground to shut the turbine off after the early morning accident and could not get out in time, according to officials. The slippers belong to Joy Sing Tisso (59), the police said.

"The rescue operation has been slow," complained Biren Balmiki, whose father Prempal Balmiki (54), a sanitation worker with NEEPCO is among those trapped in the power station of the 275 MW plant.

Balmiki arrived from Badaun in Western Uttar Pradesh in the early 1980s to work at the plant, NEEPCO's maiden venture. Like Love John and his siblings, Biren and his brother Manoj have been spending their days at the accident site watching the workers and waiting. "For the last four days I have been here," Biren said.

Sreejith T, the Superintendent of Police, Dima Hasao said a team of the NDRF is on the job and they are being helped by the State Disaster Response Force personnel. "There are as many as 20 members of the NDRF who are being helped by 60 personnel from the SDRF," said Sreejith T adding that the rescue may have to continue for another day or two.

Rescue operation could only start after October 12, when the NEEPCO engineers with help from experts from Bhakra Beas Management Authority and NHPC could finally close the intake gate of the tunnel stopping the flow of water from the reservoir. "It was not easy. The reservoir's capacity is six trillion litres," said Vinod Kumar Singh, Chairman-cum-Managing Director, NEEPCO.

Singh said the rescue operation is on in full swing and it has faced difficulties because the power station is packed with machines under the debris.

"As many as Rs 300 crore worth of our machines must be under the debris. In all it must not be less than 800-900 truckloads of debris," Singh said explaining how this is blocking the access and creating difficulty for the rescuers.

As many as 300 workers are directly or indirectly involved including 90 workers who are just engaged in clearing the muck, according to NEEPCO. Pumps and earthmovers have also been pressed into service.

Singh blamed the illegal rat-hole coal mining in neighbouring Meghalaya which has polluted the Kopili river as the reason for the accident. "One and only reason (for the accident) is the acidic water," said Singh claiming the Ph level of the water in the reservoir was 3.25 "very close to sulphuric acid."

He said over the last decade, "few hundred measures costing around Rs 100 crore" were taken to mitigate the problem. Singh claimed there was no negligence on the part of the officials and all protocols were followed and all possible agencies approached on the subject.

According to present estimates by NEEPCO officials, the damage to the hydel project could be worth around Rs 500-600 crore. "This assessment could vary drastically later," Singh said adding that the 200 MW part of the 275 MW project has suffered the damage.

"The project will only start after total renovation and modernisation," Singh said adding that he has told the Centre that in case of projects like Kopili which face problem of acidic water the 35-year-rule for renovation and modernisation should not apply. "The corrosion and damage is faster in these plants," he said.

Meanwhile, Sum Ronghang, Assam government's Hill Area Department Minister said only a thorough inquiry will help go to the bottom of the mishap. "We need a proper inquiry. It is a serious incident," said Ronghang, who was directed by Chief Minister Sarbananda Sonowal to lead a high level team to the site of the accident. "I will be submitting my report in a couple of days," he said.