The suspects face up to 15 years in prison if convicted on charges of involuntary manslaughter and participating in organised crime.
18 Afghan migrants and refugees were found dead inside a truck abandoned on a dirt road near the capital Sofia.
Prosecutors said the truck was left near the village of Lokorsko after the driver and his companion found that many of the 52 people in the hidden compartments of the truck, which were isolated with foil, were dizzy and some had already died.
The discovery on Friday was the deadliest incident linked to people smuggling in Bulgaria as the country struggles with a surge in illicit border crossings.
The truck was transporting the Afghans who had arrived from Turkey and were heading towards Western Europe via Serbia, initial investigations showed.
Hristo Krastev, a spokesman for the Sofia Public Prosecutor’s Office, told reporters that the six Bulgarians indicted in the case included the alleged ringleader of the smuggling group.
They face up to 15 years in prison if convicted on charges of involuntary manslaughter and participating in organised crime and people smuggling.
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.
Investigators said the truck’s drivers heard loud noises and knocks coming from the back but only stopped later, when they discovered the dead victims 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,” Sarafov told reporters.
Some of the 34 people who were rescued remained in hospital, some for carbon monoxide poisoning from inhaling exhaust pipe gases.
The victims, believed to be aged between 13 to 35, had paid up to 7,000 euros ($7,500) each to the smugglers, 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.
The hiding places were lined with aluminium foil to make the migrants’ presence undetectable to thermal cameras, he added.
Bulgaria, an EU member that serves as a gateway for many refugees and migrants hoping to enter the bloc, has been trying to tighten security to stop a rising number of people seeking to cross via its southern border with Turkey.
The Balkan nation has also faced mounting accusations of abuses by security forces trying to stop people from entering, with asylum seekers saying they have been pushed back, locked up, stripped and beaten.
Bulgaria has denied the allegations.
In 2015, three Bulgarian truck drivers were arrested and later charged with the deaths of 71 migrants found dead beside an Austrian motorway.
In October 2019, British police found the bodies of 39 people inside a refrigerated container that had been hauled to England. Police said all the victims, who ranged in age from 15 to 44, came from impoverished villages in Vietnam and were believed to have paid smugglers to take them on a risky journey to better lives abroad.
Police said they died of a combination of a lack of oxygen and overheating in an enclosed space. The truck discovered in the town of Grays, east of London, had arrived in England on a ferry from Zeebrugge in Belgium.