Bulgarian authorities on Friday found the bodies of 18 people who suffocated in a truck smuggling them through the country.
Bulgaria's deadliest such incident comes as the Balkan nation struggles with an increase in illicit border crossings.
The truck was transporting 52 migrants "hidden under some wood", officials said.
Locals alerted the police, who found the truck near the village of Lokorsko, 20 kilometres (12 miles) northeast of the capital Sofia.
Investigators discovered a gruesome scene with bodies scattered on the grass around the vehicle.
"According to initial reports, they died from asphyxia -- too many people in too small a place," Deputy Prosecutor General Borislav Sarafov told AFP.
"Given the number of victims, this is the deadliest incident with migrants in Bulgaria," he added.
Sarafov said the victims had died 10 to 12 hours before their bodies were recovered, with the smugglers fleeing once they noticed the deaths.
Four Bulgarians have been detained as suspects in the case.
Thirty-four people found alive were taken to hospital.
"They were cold and drenched, and they certainly had not eaten for a few days," Health Minister Asen Medjidiev told journalists earlier.
The health ministry said the dead included a child thought to be six or seven years old, but Sarafov said the youngest victim was a teenager.
Based on initial information, those on the truck originated from Afghanistan, he added.
Coming from Turkey, the group "entered Bulgaria a few days ago", crossing the border fence in the southeastern Yambol region, Sarafov added.
- Rising numbers -
EU-member Bulgaria, which serves as a gateway into the bloc, has been trying to tighten security to stop a rising number of people seeking to cross the border.
The country has stepped up controls along the 234-kilometre (145-mile) barbed wire fence covering almost the entire border with Turkey.
Border police thwarted 164,000 "irregular crossing" attempts in 2022, compared to 55,000 in 2021, interior ministry figures show.
Austria and the Netherlands in December blocked Sofia's bid to join the Schengen border-free zone.
Bulgaria has faced mounting accusations it is abusing people trying to cross over from Turkey, with asylum seekers saying they have been pushed back, locked up, stripped and beaten.
Bulgarian authorities have repeatedly denied the accusations.
Three police officers died when vehicles smuggling people rammed their cars last year.
Sofia has asked the EU for two billion euros ($2.1 billion) to reinforce the border fence and improve surveillance, but Brussels has so far refused.
Friday's gruesome discovery drew comparisons to previous cases.
In August 2015, at the peak of Europe's migration crisis, the bodies of 71 migrants, including a baby girl, were found piled up in the back of a poultry refrigerator lorry left in Austria.
A Hungarian court has jailed an Afghan national and three Bulgarians for life over the case.
In 2019, 39 Vietnamese migrants were found dead in a refrigerated truck in Britain shortly after it had crossed the Channel from mainland Europe.
Several similar but less deadly incidents have been recorded in recent years, including in Croatia, Ireland, Italy and the Netherlands.