Scores of people are missing after water and debris tore down a narrow mountain valley, smashing into the town of Dharali in Uttarakhand state on Tuesday.
Several people could be seen in videos running before being engulfed as waves uprooted entire buildings, leaving others smothered in freezing sludge.
At least four people have been confirmed killed, but at least 50 others are missing.
Government officials said shortly after the disaster that the flood was caused by an intense "cloudburst" of rain.
However, experts assessing the damage suggested that it was only the final trigger, adding to days of prolonged rains that had already soaked and loosened the ground.
P.K. Joshi, of New Delhi's Jawaharlal Nehru University, an expert on Himalayan hazards, said it appeared the flood was caused by the collapse of debris -- known as moraine -- that had dammed a lake of meltwater from a retreating glacier.
"Given the persistent rainfall over preceding days and the sudden discharge observed, a glacial lake outburst flood (GLOF) or collapse of a moraine-dammed lake is suspected as the primary trigger," Joshi told AFP.
That would have contributed to a "sudden high energy flash flood", he said, noting that glacial terrain upstream of the town included "unstable sediment zones".
Cloud cover has obstructed satellite imagery to check for the exact source of the debris, and Joshi cautioned that there was not enough satellite data for a "definitive confirmation".
- 'Disaster severity' -
Safi Ahsan Rizvi, an adviser to the National Disaster Management Authority, also said that it was "likely" that the cause was a "glacio-fluvial debris landslide".
Sandip Tanu Mandal, a glaciologist at New Delhi's Mobius Foundation, also pointed to the "possibility of a GLOF", caused by "significant water accumulation in the lake due to increased melting and rainfall".
Mandal noted that while heavy, the amount of rain immediately before the flood was "not very significant" in comparison to the vast volumes of water that poured down the valley.
That would indicate the source was a potentially collapsing lake.
Himalayan glaciers, which provide critical water to nearly two billion people, are melting faster than ever before due to climate change, exposing communities to unpredictable and costly disasters, scientists warn.
The softening of permafrost increases the chances of landslides.
Joshi said the latest disaster "highlights the complex and interconnected nature of Himalayan hazards".
Rapid development and building downstream meant that the damage caused was multiplied.
"The land use patterns in the floodplain exacerbated the disaster severity," Joshi said.
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