Six people have been charged after the bodies of 18 Afghans were found in a lorry in Bulgaria.
All six are Bulgarians, including the alleged ringleader, Hristo Krastev, and one of the suspects is on the run, a spokesman for the Sofia Public Prosecutor's Office said on Saturday.
They face up to 15 years in prison if convicted on charges of involuntary manslaughter and participating in organised crime and people smuggling.
The discovery on Friday of 18 bodies in a lorry abandoned near Sofia was the deadliest incident linked to people smuggling in Bulgaria as the country struggles with a surge in illicit border crossings.
The lorry was transporting 52 Afghans, who had arrived from Turkey and were heading towards Western Europe, investigators found.
The drivers heard loud noises and knocks coming from the back but only stopped later, when they discovered the dead and fled, Deputy Attorney General Borislav Sarafov said.
“The people transported were curled up and pushed against each other like in a tin can … They died slowly and painfully for 30 to 60 minutes. It is an extraordinary human tragedy,” Mr Sarafov said.
Some of the 34 people who were rescued remain in hospital, some with carbon monoxide poisoning from inhaling exhaust pipe gases.
The victims, believed to be aged between 13 to 35, had paid up to €7,000 ($7,500) each to the smugglers, Mr Sarafov said.
“It was out of greed that the smugglers carried 52 people. They had previously transported between 25 and 35 people at a time, at least twice a month,” he said.
Hiding places in the lorry were lined with aluminium foil to make the migrants' presence undetectable to thermal cameras, he added.
One of the suspects remains at large and was indicted in absentia, while two others who had been arrested are not expected to be charged, prosecutors said.
Bulgaria, an EU member that serves as a gateway for many migrants hoping to enter the bloc, has been trying to tighten security to stop a rising number of people seeking to cross its southern border with Turkey.
Bulgaria has stepped up policing along the 234km barbed wire fence across almost the entire border with Turkey.
Austria and the Netherlands have blocked Sofia's application to join the Schengen border free zone.