London (CNN) The deaths of 39 people found in a truck in a UK industrial park this week has sparked horror and revulsion around the world.

Little has been publicly revealed about who they were, though police said Thursday they believed the victims were all Chinese nationals. Even less clear is how and why they came to be transported across the world in what is believed to be a refrigerated truck.

A murder investigation has been launched, the Belgian Federal Prosecutor's office has opened an investigation, and UK Prime Minister Boris Johnson has called for those involved in the deaths to be "hunted down and brought to justice."

But one question has perplexed many: why would citizens from the world's second-biggest economy travel -- either voluntarily or under duress -- to the UK in such a way?

The numbers

Nearly 10 million of the international migrant population of 258 million are Chinese citizens, according to the Migration Policy Institute (MPI) -- making China the fourth-largest country of origin for international migrants.

Most of these, almost 2.5 million, reside in the US, while there are 712,000 in Canada and 473,000 in Australia.

The country's rapid economic growth in recent decades has, according to MPI, dramatically expanded China's "geopolitical and economic footprint across the world."

In the UK in 2018, for instance, more than 730,000 visas were issued to Chinese nationals -- 25% of the 2.9 million total, and a 11% increase on the previous year.

British Police forensics officers work on ther lorry found to be containing 39 dead bodies, at Waterglade Industrial Park in Grays, east of London, on October 23, 2019.

Who is migrating?

There is an assumption that migrants are often the poorest in society, seeking low-skilled employment or fleeing from terror. But this is not always true, especially for Chinese migrants.

"There's a huge range of migration from China, so you have everything from low-skilled, middle-skilled to the highly-skilled," Natalia Banulescu-Bogdan, an associate director of MPI's International program, told CNN.

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Typically, migrants traveling to Europe or the US are not the "poorest of the poor," she said, because they require significant resources to move, whether they are migrating legally or illegally.

"If you're traveling through regular channels you need to think about passports, visa fees," she said.

"If you're traveling through irregular channels, smugglers often exact quite high fees and, more than that, it's about know-how and being aware of opportunities, and this necessitates a quite sophisticated network of people abroad. Often, it's the household who've had people move abroad that are likely to move."

Police stand guard at the site where 39 bodies were discovered in the back of a lorry.

Migration patterns of the wealthy

Chinese migration has many forms. Where that country's skilled citizens choose to live differs from their lower-skilled compatriots, experts say.

"You have migration pathways for very skilled people in academia, in the science and technology sectors that might take them to the United States and other high-income countries," Banulescu-Bogdan said. "And you may have pathways for construction in Africa or eastern Europe. They are very different numbers."

According to MPI, 10 to 20% of China's migrants are in Africa and are diverse in their socioeconomic class, occupation and age, ranging from diplomats or aid workers to laborers working on infrastructure projects.

Many Chinese students study abroad, too. In the UK, almost 100,000 Tier 4 study visas were granted to Chinese citizens -- up 13% on the previous year and about 40% of the global total.

Economist Christian Dustmann, from the London-based Centre for Research and Analysis of Migration (CReAM), said many Chinese students in the UK are from wealthy families. "They are willing to pay high fees and live in expensive cities like London," he told CNN.

Italy's 'shadow' economy

According to the UK government's 2018 annual report on modern slavery , China is the fourth-most common country of origin for victims of modern slavery in the UK, behind the UK itself, Albania and Vietnam. In 2017, 293 potential victims of modern slavery in the UK were reported to originate from China, the government said.

But Dustmann added the number of illegal Chinese migrants in the UK is still small compared to those in Mediterranean countries such as Spain and Italy, mainly because it is more difficult to enter the UK.

"Of course, there are illegal immigrants from China in all European countries," he said.

"Italy is one of the big destinations. They have fueled the textile industry in Italy where they are willing to work at low wages. Italy has a large shadow economy where you find many migrants from China and the Middle East."

An economic boom that led to inequality

In the US, 1,077 Chinese nationals were apprehended by the US Border Patrol in 2018, spiking at 2,439 in 2016. So why are so many Chinese citizens willing to take such risks?

In China, vast migration from rural to urban areas powered the economic boom, while pension reforms also helped move Chinese citizens out of poverty. "China has developed dramatically, poverty has decreased dramatically, the economic circumstances have improved -- whether that will continue at that pace in the future is questionable," Dustmann said. "Everything I've seen over the past years was actually positive, but that doesn't mean that there were some groups who may have suffered more than others."

Banulescu-Bogdan said that in becoming wealthier, China became a less equal society. "The opportunities within a country as large as China are not equally distributed," she said. "You do have the high end of the spectrum, but you also have people in more desperate circumstances."

Trafficking and smuggling

According to the Migration Policy Institute, Chinese citizens have migrated for many reasons over the years, including political repression, the one-child policy and a desire to study abroad.

But the methods to leave can vary significantly. "The opportunities that are available are not available equally for all citizens and this is also where smuggling and trafficking comes in, particularly with human trafficking," Banulescu-Bogdan said.

"We don't know yet if this was a smuggling operation gone wrong, but it's important to understand that there's no white line between the two.

"A young man who wants to find employment opportunity aboard might, for instance, engage the service of a smuggler from point A to B but, along the way, the relationship could turn more coercive.

"A criminal syndicate could attract more money from these people, or there could be multiple legs getting from China to the UK and it's not one group, the migrants get handed off. It's possible some of the drivers along the way didn't know they were carrying human cargo. There's a very wide range in terms of the criminality behind these movements."

This story has been updated.