More than a week after a glacial collapse near the Langtang Lirung mountain peak set off devastating floods in Nepal and Tibet, sending a deluge of rocks, mud and water to the valleys below, forensic experts are working to identify bodies.

More than 1,270 people have been killed, and more than 4,700 are still missing, authorities said on Thursday. On Friday morning, reports emerged that two of the hundreds of workers trapped in tunnels at hydropower projects along the affected rivers had been rescued.

Meanwhile, authorities are using DNA testing to try to identify hundreds of bodies that have left morgues overflowing. Officials are urging first-degree relatives – fathers, mothers, sons, or daughters – of those reported missing to provide fresh blood samples for DNA profiling, police have said.

For missing foreign nationals, a first-degree relative (also a father, mother, son, or daughter) abroad can also undergo DNA profiling in their home country and email the results to Nepal Police for comparison. Hundreds of DNA samples have so far been collected from relatives.

The samples collected are being sent to the Nepal Police Forensic Science Laboratory and the Nepal Academy of Science and Technology.

So why is DNA identification important in the aftermath of the floods and how does it work?

Why is DNA testing needed?

In Nepal’s capital, Kathmandu, forensic teams are working to identify bodies – but many have been found in a state of decomposition, said Al Jazeera’s Jessica Washington, reporting from a DNA testing facility in the city.

“Forensic teams on the ground told us that there are some things that can help with the identification process, such as tattoos and jewellery, but many bodies were found in such a state that it is very challenging to identify them,” said Washington.

Decomposition is why DNA testing becomes essential, say experts: mutilated bodies often become unrecognisable, especially after prolonged submersion in water.

DNA profiles can be generated from blood, soft tissue, bone and teeth. According to Kerstin Montelius, a molecular biologist at Sweden’s National Board of Forensic Medicine, DNA material can still be extracted from bone even if most of the tissue has decomposed.

“Visual identification might be impossible at an early stage of the process, with confusion as a result,” Montelius told Al Jazeera. But, she cautioned, “DNA degrades, too, and circumstances set the decisions.”

How does DNA testing work?

The globally recognised framework for managing and identifying disaster victims is Interpol’s Disaster Victim Identification (DVI).

DNA molecules contain the information all living cells in the human body need to function, and apart from identical twins, who share almost the same DNA, each person’s DNA is unique. As such, it can be used to identify disaster victims or missing people.

As a part of the DVI process, human remains are inspected by specialists to detect as much biometric data as possible.

“There are three primary methods of identification that have shown to stand by themselves with a high score of probability in the identification process: DNA comparison, fingerprints and teeth comparison,” Montelius said, adding that the method chosen depends on the circumstances.

When a direct DNA profile of the missing person is available – for example, a missing person’s DNA sample can be taken from personal items such as toothbrush or hairbrush – the unidentified body’s DNA profile is compared with that of the missing person to see if they match.

However, sometimes there are “no fingerprints of the victim anywhere, or it could be difficult to find information on the teeth at the dentist’s”, said Montelius.

In such cases, DNA profiles can also be collected from victims’ next of kin, said the molecular biologist. In Nepal, that is being done through blood samples.

Once the data is collected, a team of specialists compares and reconciles the two sets of information – from the next of kin, as well as the human remains found – to identify the victims.

This can be done in one of several computer systems on the market, with the system able to suggest matches for the DNA expert to scrutinise, said Montelius.

Identification can be declared if there is a 100 percent match between the two sets of data.

Are other countries helping?

India has sent a 19-member forensic team that began a DNA sample analysis alongside their Nepali counterparts at the Nepal Police Central Forensic Science Laboratory and the National Forensic Science Laboratory, according to the Indian embassy in Nepal’s capital.

“The team is working to expedite the processing of DNA samples in Nepal for the identification of those affected by the recent flash floods,” the embassy said.

China has also sent a second contingent of DNA identification experts, alongside a third batch of emergency relief supplies, the state-run Xinhua news agency reported on Thursday.