It’s a case that has perplexed officials across the world: On October 23, paramedics discovered a refrigerated truck container holding 39 dead bodies in an industrial park outside of London. How and why they ended up there is unclear. Over the past few days, officials from at least five countries have been trying to gather more information. So far, three people have been arrested, including the truck driver, on suspicion of murder. On Friday, police arrested three other individuals on suspicion of conspiracy of human trafficking and manslaughter, indicating that the deaths may be related to human smuggling, the New York Times reports.

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But as of now, officials have more questions than they do information: Where did these people come from? What happened to them? And who’s to blame? Here’s everything we know so far.

At approximately 1:40 a.m. on Wednesday, paramedics called police to the Waterglade Industrial Park in Grays, England, where they had discovered the bodies of 39 people — 31 men and eight women. All were pronounced dead at the scene.

Right now, officials from Belgium, Britain, Bulgaria, China, and Ireland are trying to determine the timeline of the truck and container’s movements. Investigators believe that the truck — which was registered in Bulgaria — started its journey in Northern Ireland and entered the U.K. on Sunday. According to Belgian officials, the container arrived in the Belgian port of Zeebrugge on Tuesday afternoon before making its way to Purfleet, where the truck collected the container. Currently, investigators don’t know when the victims entered the container, but it seems likely that they suffocated inside.

On Thursday, Essex Police announced in a statement that the 39 victims are believed to have been Chinese nationals. However, this statement has been contested. On Friday, human-rights activist Hoa Nghiem tweeted that the family of a Vietnamese woman, Pham Thi Tra My, 26, was worried that she was among the deceased. According to her family, My had been attempting to travel to Britain, but had encountered roadblocks along the way, the Times reports. The last the family heard from her was late on October 22, when she sent her mother the text, “I’m sorry Mom, my path to abroad didn’t succeed. Mom, I love you and Dad so much! I’m dying because I can’t breathe.”

Upon arriving at the scene on Wednesday morning, police arrested the truck driver, a 25-year-old man from Northern Ireland, on suspicion of murder. Per CNN, a local councillor later identified the man as Morris Robinson. However, according to The Telegraph, sources close to the investigation say it’s “very unlikely” that Robinson knew of the container’s contents. It’s believed that Robinson called paramedics after opening the container and discovering the bodies, though this detail has not been confirmed. Robinson has not yet been charged.

On Friday morning, Essex police announced that they had arrested two more people: a 38-year-old man and a 39-year-old woman from Warrington, in northern England, on suspicion of conspiracy to traffic people and manslaughter. Neither have been publicly identified. Then, hours later, a 48-year-old man from Northern Ireland was arrested at Stansted airport on the same suspicion, the Guardian reports.

Police haven’t named a motive, but the three most recent arrests suggest that the murders were related to human smuggling. According to the Times, while there’s a dearth of reliable statistics regarding informal migration out of China, many Chinese nationals leave with the goal of attaining political asylum in a Western country. Sometimes, these people opt to pay a middleman — known as a “snakehead” — to orchestrate the trip. But this course of action is a particularly dangerous one. In fact, in 2000, 58 Chinese migrants died from asphyxiation in a refrigerated truck that was discovered in Dover.

But Hua Chunying, China’s Foreign Ministry spokesperson, hasn’t been so quick to accept this narrative. During a news conference on Friday, she argued that people in China were happier than ever and wouldn’t be fleeing the country in such a way, per the Times.

“The Chinese people’s sense of happiness, security, and satisfaction is unprecedented,” she said. “If you look around the world, there are many countries with illegal immigration issues. China is not the one with the serious problem.”

This post has been updated.