Eighteen migrants were found dead in an abandoned truck, the interior ministry said on Friday, as the Balkan nation grapples with an increase in illegal border crossings.
The vehicle was “carrying around 40 migrants hidden under a tree”, the ministry said. “Eighteen are dead.”
After locals alerted the police, the truck was found near the village of Lokorsko, 20 kilometers (12 miles) northeast of the Bulgarian capital, Sofia.
Some of the migrants who were still alive in the truck were taken to hospital.
A search is underway for human traffickers who are believed to have driven the truck and fled, authorities said.
It is believed to be the biggest incident involving migrants in Bulgaria.
EU member Bulgaria, which is the gateway to the bloc, has been trying to tighten security to prevent large numbers of people trying to cross the border.
In an effort to stem the flow, Bulgaria has stepped up control along a 234-kilometer (145-mile) barbed-wire fence that covers 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 widespread accusations of mistreatment of people trying to cross from Turkey, with asylum seekers saying they have been deported, locked up, stripped and beaten.