Eighteen migrants have been found dead in an abandoned truck, the interior ministry said on Friday, as the Balkan nation struggles with an increase in illicit border crossings.
The vehicle "was illegally transporting around 40 migrants hidden under some wood", the ministry said. "Eighteen of them have died."
After locals alerted the police, the truck was located near the village of Lokorsko, 20 kilometres (12 miles) northeast of Bulgaria's capital Sofia.
Some of the migrants still alive in the truck were taken to hospital.
A search was underway for human traffickers believed to have driven the truck and fled, authorities said.
It was believed to be the deadliest incident involving migrants in Bulgaria.
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.
In a bid to stem the flow, Bulgaria has stepped up controls along the 234-kilometre (145-mile) barbed wire fence covering almost the entire border with Turkey.
Austria and the Netherlands have 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.