The final minutes before floodwater crashed through Nepal-China border

The video lasts only seconds.

A dark wall appears from behind the buildings at Gyirong, on the Chinese side of the Nepal-Tibet border. At first, it looks almost like a black cloud. Then it gathers shape and speed: water, mud, boulders, ice and rock surging down the valley. The CCTV footage, whose timestamp reads 10:59:53, went viral.

For many, it was the first visual evidence of the sheer scale of the destruction that was unfolding.

What happened in the minutes that followed is still difficult to reconstruct. But fragments of video and satellite imagery show much of the border complex swept away, with buildings that once crowded the valley floor simply gone.

Electronic immigration records from Nepal's Rasuwagadhi border post are filling some gaps, as well as insights from its only surviving staff member. Tika Ram Pokharel, who happened to be in Kathmandu sitting an exam on the day, tells the BBC the crossing was unusually busy when floodwater hit.

"This is a historic travel route between Kathmandu and Tibet. The area is geologically vulnerable, characterised by deep gorges and steep rivers that flow at high velocity, carry heavy sediment and can bring down huge boulders," says Ajay Dixit, a Kathmandu-based hydrologist.

It is a place where pilgrims heading towards Mount Kailash cross paths with traders travelling between Nepal and China, truck drivers, customs officials, tourists and people who live along the border.

On the morning of the flood, more than 100 Indians may have been in or around the crossing. At least 275 Indians remain missing in Nepal, according to India's foreign ministry.

Nepal's immigration department has been trying to reconstruct who passed through Rasuwagadhi in the final hours before the disaster.

Electronic records show that 89 Nepali and foreign nationals had cleared immigration to leave Nepal for Tibet, while four people had registered their entry into Nepal.

The immigration system recorded its final transaction at 08:35:03 local time (10:52 Tibet time), less than 10 minutes before the torrent appeared on the CCTV footage at Gyirong.

Four immigration employees, including the head of the Rasuwagadhi office, went missing in the floods. His body was later recovered far downstream. The female official who recorded the final transactions is still missing.

Among the foreign nationals recorded leaving Nepal that morning were 32 Indians, 12 Chinese nationals and nine Americans, as well as citizens of Britain, Canada, Australia and Italy.

Tika Ram Pokharel, the surviving senior immigration assistant at Rasuwagadhi, says the electronic records capture only part of what was happening at the border that morning.

The immigration office stood at Ghatte Khola Bazar, about 1.5km north of Timure, a small trading settlement clustered around the crossing. From there, it was another 2km to the Friendship Bridge marking the border with Tibet. Along that stretch were shops, hotels and parking areas where travellers and vehicles gathered before crossing.

The Nepalese immigration office opened at 08:00, Pokharel says. Once travellers had cleared immigration, they could continue towards the border or wait in the area before crossing.

When Pokharel called a colleague at 08:20 that morning, he was told the crossing was unusually busy. Around 300 to 400 people were waiting to leave Nepal for Tibet, he says, while others arriving from the Chinese side were also in the area.

"Those who completed the immigration formalities that morning, I don't think were able to complete their immigration process on the Chinese side," Pokharel tells the BBC.

Pokharel says it took 15 to 20 minutes to walk from the Nepalese immigration office to the bridge after travellers had cleared immigration.

Travellers arriving late the previous afternoon sometimes had to wait until the border reopened the next morning, while those travelling in groups or by bus would often wait for everyone to clear immigration before moving on together.

That makes it difficult to know where those 89 people were when the flood struck.

At 08:37 - less than two minutes after the immigration system recorded its final transaction - seismic instruments detected movement in the mountains above the border, according to a timeline compiled by Nepal's Centre of Hydrology and Water Resources Research. Researchers later attributed the signal to the collapse of part of a glacier that triggered the flood.

China said the mudslide reached the border crossing from its origin in around seven minutes.