It's the job of a forensics team in Arizona to identify the bodies of migrants found in the desert. Anthropologist Robin Reineke describes how she pieces together the sad jigsaw puzzle of personal attributes and belongings.

There are many ways to enter the US. The way that's taken by the very poorest is to come through the Sonoran Desert on foot.

It's a very forbidding place - the temperatures in the summertime are regularly in the triple digits Fahrenheit and there's no water.

Groups of people will walk for three to five days, travelling by night and drinking out of cattle troughs or whatever they can find. It's unlikely they will see other people for the duration of the trip.

At present, Tucson, Arizona has almost 800 unidentified bodies of migrants who didn't make it to the other side.

They were found by border patrol, by citizens of the Tohono O'odham Indian reservation, or by ranchers or hikers. I compare these unidentified remains to lists we have of missing persons, collected from families of migrants last known to have been attempting the crossing.

When there is a possible match, I call the family.

Recently, I called the wife of a missing man. "There's someone who could potentially be your husband," I told her. "Can I ask you some questions?

"Did he have any tattoos? You said that he was missing an upper molar. Could you tell me more about that?"

One day in the desert heat is enough to make a body unrecognisable, so the possessions that are found with the remains can be incredibly important to the family.

There's often an interesting combination of objects. Mostly it is the normal stuff that anyone would take with them on a trip - toothpaste, socks, snacks, water.

But then there are these very personal items - photographs of loved ones, handwritten notes from family members, kids' drawings.

The letters are from the children or wives of those we've found dead, wishing them luck and telling them that they're loved, that they should be very careful on the journey, that the family's prayers are with them, that the family's hopes are with them.

And the photos have been touched and pulled out over and over again, then folded up and put back carefully.

Pima County Missing and Unidentified Persons Project Robin Reineke and a forensic anthropologist, Bruce Anderson, set up the project in 2006 to solve "cold cases" of unidentified migrant bodies

Last year alone, 150 bodies of migrants were discovered - 66 of these remain unidentified

They have records for over 1,300 missing migrants last seen crossing the US-Mexico border

The project was recently able to upload all missing migrant cases into the US Department of Justice website

Some of the items have unspoken stories.

There was a young kid - he was probably only 15 or 16 years old - and the soles of his shoes were just completely worn off. He had been carrying one orange paper flower.

I remember a man who had a small dead hummingbird in his pocket. I know that for a lot of indigenous North American peoples hummingbirds hold a sacred significance - they represent hope and love and they're a powerful protective symbol.

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On Prayer card Wallet Money False ID List of names and numbers SIM cards Money Many migrants carry all the money they have. Before they meet up with their coyote (guide) they are often carrying the payment with them in cash, which is usually more than $2,500. This is very dangerous. Every year, hundreds of Central American migrants fall prey to criminals, especially drug cartels and gangs. False Mexican Voter ID card When false Mexican ID is found with unidentified remains, Robin’s team look at their missing lists from neighbouring countries – in this case, the owner was Honduran. He would have needed false ID to travel through Mexico, where he would also have been considered an “illegal”. Another reason for the fake ID is that if he had been caught trying to cross into the US, he would only have been deported back to Mexico rather than all the way to Honduras. SIM cards These are not uncommon. Migrants often carry them as back-up in case their cell phones get taken. List of names and numbers This is one of the most common items found with migrants. The guides often take their cell phones at the start of the journey, so the migrants carry numbers with them instead. The numbers were key to helping the team identify this individual, who was found in January 2011, about 10-15 months after his death. Someone from the Mexican consulate called one of the numbers and spoke to a person who knew a man using false ID. He was ultimately identified through a DNA match, thanks to the Argentine Forensic Team, which funds a large number of DNA comparisons for Central Americans. Prayer card The words read “Cristo mi piloto” (Jesus is my pilot) with a prayer on the back. Wallet Since all travellers carry wallets, and many of them are very similar, these rarely help in the identification process.

With certain objects, my familiarity with Mexican and Central American cultures helps me to make a guess about where someone came from.

For example, many migrants carry prayer cards - small cards with a saint or a holy scene printed on them with an accompanying prayer. A prayer card of the Virgin of Juquila is likely to have belonged to a Oaxacan traveller, since it is there that she is venerated.

You can think of it like a puzzle - a puzzle which has a great deal of importance to a lot of people.

Case ML #10-00533 The owner of these prayer cards was found by an off-duty border control guard while he was hiking on a remote mountain, on 12 March 2010

The male, aged between 22 and 35, had likely died years earlier, since little more than his skeleton remained

While most anthropologists who work in human identification are physical or forensic anthropologists who specialise in skin and bone, I am a cultural anthropologist focused on the social world.

My training helps me bridge the gap between forensic scientists and the families of the missing, who are generally indigenous people.

The forensic scientists and I have a lot of faith in science, which is a belief system. It requires trust in officials, in the scientific method, in technology. Many families of missing migrants are coming from communities in Mexico or Central America that have been violated for centuries by officials, scientists and technologies. There is a lot of mistrust.

About the photographer Jonathan Hollingsworth is a documentary photographer in New York City

His book Left Behind: Life and Death Along the US Border is available from Dewi Lewis Publishing

Moreover, the family is looking for something recognisable, something that carries the trace of the person they knew.

They can get fixated on details scientists might consider nearly irrelevant, like the buttons on a pair of trousers or a brand of tennis shoes. These are things that have meaning to them. They are also things that someone can change, unlike their DNA or fingerprints.

So while I might be trying to arrange a DNA comparison with a family, they'll be asking to see photos of the remains. Sometimes they show up in my office, having driven for hours, asking to see the body.

Case ML #11-01516 Skeletal remains of a male aged between 14 and 19 were found on August 5, 2011

The team phoned the number noted down on a scrap of paper labelled "mi mama" - it was indeed the mother of a missing young man

Generally, we don't allow this unless the dead person is recognisable. Some might argue that it is not our job to protect families from the truth. But if they see an unrecognisable, decomposed corpse or skeleton, they may not only be traumatised, they may simply reject the identification. I've been involved in a few of cases where we have come very close to a positive ID and the family suddenly rejects involvement or says we're lying.

Because of the highly decomposed nature of the bodies, the calls I make are never as simple as, "I am sorry to inform you…" Instead, it's the beginning of a process that could take months. It unfolds as a kind of a negotiation between the scientists and the families. Both sides have the same goal - to find the missing person.

But for the scientists, the problem is an unidentified dead body, whereas for the families, the problem is a missing living person. These realities pull them in opposite directions.

And, coming after a long period of not hearing anything, these are months of agony for families. What they have already endured is terrifying. You cannot grieve without a body - without certainty that the person is gone. Every single day that you are living a normal life, you know they could be suffering.

Case ML #10-01482 The owner of the possessions above was found on 25 July 2010, partially suspended from a tree by his shoelaces

He was part of a large group of Guatemalans attempting to cross the border that summer

Reineke made a trip to Guatemala to meet the families of the missing, but this man has never been identified

Image caption The family of Martin Mejia Gomez, who is missing. He was travelling with his 14-year-old cousin whose body was found and identified

One case that stands out is a large group of Guatemalans that were all from the same town, who attempted to cross in the summer of 2010.

The man who was leading the group had crossed before, maybe five or 10 years earlier. But the border had completely changed - it is a lot more difficult to cross over now. The man got lost with this whole group of friends and relatives following him.

One of them was able to call his wife and tell her that they were lost, that they had been walking for seven days, that they were out of food and water, that some of the group had been vomiting blood, that they knew they were going to die.

I received all the reports of the missing and a big sleuthing operation began. Three sets of bodies were found in different locations, months apart. We were able to identify five of them. The other seven or eight remain missing.

I went to Guatemala with my husband in the summer of 2011 to meet the families of the dead and missing from this group. I was struck by the difference between those whose loved ones had been identified and those who were still waiting.

Even with the knowledge that the whole group had been lost in the desert, and that some of them had come home in coffins, the families of the missing were distraught - sickened by the condition of not knowing. Their faces are burned into my memory.

Mexican Migration into the USA Around 12m people have immigrated to the US from Mexico in the last 40 years

Just over half of these were unauthorised immigrants

In the last five years net migration from Mexico has fallen to zero because of the effect of the US recession, increased border enforcement and the falling birth rate in Mexico

In 2005, more than a million Mexicans were arrested attempting to cross the border illegally - by 2011, the figure had fallen to 286,000 Source: Net migration from Mexico falls to zero - and perhaps less, Pew Hispanic Center

I won't pretend that I have an easy job and that I don't take some of it home. I work with an incredible team, who have taught me everything. And I have a wonderful husband and a fuzzy dog at home to keep me sane.

And I feel very honoured to be doing this work. The unidentified are here in Tucson and I feel an obligation to them.

The immigration issue is tough but it's not intractable.

For me, it's frustrating that there is so much attention on the physical line of the border, and on border security. When hundreds are dying each year, their bodies rotting in the desert, we have to think about what is an effective border, what is effective security - and security for whom? It is time to really think about what we have done as a continent to cause people to risk their lives in such large numbers.

Immigrants are being blamed for a lot at the moment - they're being scapegoated. I feel that these people were, by and large, good, hard-working people that did not mean any harm. By attempting the crossing they were trying to do the very best for their families.

I would definitely do the same thing.

Robin Reineke spoke to Outlook on the BBC World Service. Listen back to the interview via iplayer or browse the Outlook podcast archive. You can follow the Magazine on Twitter and on Facebook