Rescuers flew a drone inside a tunnel in the Indian Himalayas to look for 35 construction workers believed trapped there since a surge of water and debris swept down a mountain valley destroying dams and bridges, officials said on Wednesday.

Some 204 people remain unaccounted for since Sunday’s disaster in the northern state of Uttarakhand, most of them workers at the Tapovan Vishnugad hydroelectric project or at Rishiganga, a smaller dam which was swept away in the flood.

So far rescuers have retrieved the bodies of 32 people from mountainsides and further downstream the Dhauliganga river, the state police said.

Rescuers were focused on saving people stuck in the flooded tunnel connected to the 520 mw Tapovan project, deploying heavy excavators to remove water and slush.

“We have entered the tunnel, but have not been able to go beyond 120 metres, There is water all the way up to the roof,” state police chief Ashok Kumar said.

There was also concern that those who had survived the flooding inside the tunnel were at risk of hypothermia, a dangerous drop in body temperatures because of cold conditions, said Vivek Pandey, a spokesman of the Indo-Tibetan Border Police that is also involved in the rescue effort.

A drone with five cameras was sent inside a short stretch of the tunnel on Tuesday, but it failed to showed any human presence, an official said.

The severe flooding on a bright day was initially thought to have been caused by a part of a glacier in the Nanda Devi region breaking off and crumbling into the Dhauliganga river.

But scientists say it is more likely that there was an avalanche caused by the melting of snow in warmer weather which led to the flash flood.

“What happens in an event like this is that if there is a sudden change in temperature, the fresh snow on the surface begins to melt and slip because of the higher ambient temperature, ” said D.P. Dobhal, a glaciologist.

He said glaciers in the area contain a large amount of debris and when the snow slips, it begins to carry with it the debris. “It ultimately becomes very strong, eroding everything that comes in the way.

A team of scientists sent to the area to find out what triggered the flood in the ecologically sensitive mountains is expected to submit its report by the end of the week.