At least 600,000 soldiers who died in France during World War I are still officially missing, their resting places unknown and unmarked.

While the passage of time renders the task of recovering the lost war dead increasingly complex, it is still possible to identify a few of the fallen.

The first step to is to determine whether discovered remains are really those of a soldier from World War I.

Researchers use the state of the remains and scraps of uniform or equipment to check that the skeleton doesn't date from an earlier period or is evidence of a crime scene.

Then they try to ascertain the soldier's nationality.

"The best sources of proof are metal-reinforced leather boots, which preserve well and are different depending on the country," said Stephan Naji, head of the recovery unit at Commonwealth War Graves Commission (CWGC).

His team in the Calais region of northern France is regularly contacted when remains are discovered.

Soldiers who are uncontestably French or German are handed over to France's War Veteran's Office (ONAC) or Germany's VDK war graves agency.

"If there's a military plaque with a name of it and proof of next of kin, the soldier's descendants can repatriate him to his family home or they can let the state bury him in a national cemetery," said ONAC's Stephane Jocquel.

DNA tests are seldom carried out on the remains of French combatants.

One of the CWGC's missions is to help the authorities identify as many as possible of the 100,000 soldiers from the former British Empire who are still missing.

Buttons and insignia from uniforms are key clues, as are regiment badges as well as water bottles or whistles bearing the name of the soldier's unit.

But all the tell-tale signs need to tally. Some soldiers swapped badges as a sign of comradeship or recovered equipment from fallen brothers in arms. Australian boots, for example, were particularly prized for their quality.

Investigators also clean personal items, like razors, forks and watches, for fine details like the owner's engraved initials or a hallmark indicating the date and place the object was made.

If they can confirm the soldier's nationality, they pass on the information to the country's authorities, who cross check it with their lists of missing combatants.

Some countries, including the United States, Australia, Britain and Canada, carry out genealogical research to try to trace descendants, including DNA tests if any are found.

At the Department of Defense, one division works to bring home the tens of thousands of unidentified soldiers. At the Defense POW/MIA Accounting Agency, experts spend years using DNA, dental records, sinus records and chest X-rays to identify the remains of service members killed in combat, CBS News reported last month.

Since 2015, the DPAA has identified nearly 1,200 soldiers, sailors, airmen and marines using remains returned from 45 countries.

In 2006, the remains of U.S. Army Pvt. Francis Lupo was the first World War I casualty to be recovered and identified by the agency.

Last year,  British and Canadian authorities gave seven soldiers killed in World War I a full military burial after their remains were discovered during a gas pipeline construction in Belgium.

The search can take several years and is successful in only about 2-3% of cases, according to Alain Jacques, head of the archaeology service in Arras, northern France.

If a soldier is successfully identified, his remains are buried with military honours at the nearest Commonwealth cemetery, in the presence of descendants who wish to attend.

When the soldier cannot be identified, he is reburied with honors under a gravestone bearing the words "Known unto God."

The epitaph was chosen by British poet Rudyard Kipling, who spent years fruitlessly searching for his own son after he went missing, aged 18, in what would be called the war to end all wars.

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• World War I