The vehicle was transporting timber and carrying some 52 illegal migrants hidden in a secret compartment, the country’s Interior Ministry said.
The truck was transporting 52 illegal immigrants from Afghanistan
The vehicle was found near the village of Lokorsko, around 12 miles from the Bulgarian capital, Sofia.
Heart-rending pictures show shoes and other belongings from the migrants strewn around the claustrophobic van.
It is believed to be the deadliest incident to date involving migrants in Bulgaria.
The drivers, suspected to be human traffickers, had fled the scene.
Senior police chief Atanas Ilkov said one of the four people detained had already been sentenced for human trafficking, and charges would be made once there was sufficient evidence.
Survivors were taken to nearby hospitals for emergency treatment.
Bulgarian health minister Assen Medzhidiev has said the survivors were in a precarious condition.
He confirmed: “They have suffered from lack of oxygen, their clothes are wet, they are freezing, and obviously haven’t eaten for days.”
Bulgaria is located on a popular route used by migrants from countries in the Middle East to enter the EU.
The Balkan nation is typically a stepping-stone to richer countries in western Europe, used often by elaborate networks of smugglers.
Last year, more than 150,000 people attempted to cross the border with Turkey, nearly five times higher than the previous year.
The country has long struggled to deal with the number of people trying to enter the European Union through its borders.
Bulgaria has also faced accusations it is mistreating people trying to cross the border, with asylum seekers claiming they have been stripped, beaten, and arrested.
TheNewsGuru.com (TNG) reports that this came three years after Maurice Robinson was convicted of manslaughter for the death of 39 Chinese migrants found in a refrigerated lorry in Essex.
The victims, who had arrived at Purfleet from Zeebrugge in Belgium, had been dead for at least 12 hours before the gruesome discovery.
The truck had been abandoned for 4 days
A senior police chief has said that one person has already been sentenced for human trafficking.