Authorities in Bulgaria have detained seven people in connection with an abandoned truck in which 18 people believed to be migrants were found dead, police said.

The bodies were discovered on Friday in a secret compartment below a load of lumber in the truck, which was left on a highway not far from Bulgaria’s capital, Sofia.

Borislav Sarafov, director of Bulgaria’s National Investigation Service, confirmed that all the victims had died of suffocation. He called the case the country’s deadliest involving smuggled migrants.

Police also found 34 survivors in the truck, most of them in very poor physical condition, Bulgarian Health Minister Assen Medzhidiev said.

Police officers stand guard at the village of Lokorsko, near Sofia, Bulgaria where an abandoned truck was found containing the bodies of 18 migrants. Photo / Valentina Petrova, AP

All the passengers originally were from Afghanistan and had entered Bulgaria from Turkey while hoping to reach Western Europe, authorities said.

Sarafov said the people who died had perished 10 to 12 hours before the truck was found and that the smugglers had fled the scene after they noticed the deaths.

The seven suspects were detained at different locations across Bulgaria. Investigators were working to determine if the truck’s driver was among them.

Police at the site where an abandoned truck was found containing the bodies of 18 migrants who appeared to have suffocated to death inside a secret compartment under a load of lumber. Photo / Valentina Petrova, AP

The investigation indicates the suspects belonged to an organised crime ring involved in smuggling migrants from the border with Turkey to the Bulgaria-Serbia border, Sarafov said. Passengers paid €5000-€7000 ($8500-$12,000) each, he said.

Bulgaria, a Balkan country of 7 million and the poorest member of the European Union, is located on a major route for migrants from the Middle East and Afghanistan seeking to enter Europe from Turkey. Very few plan to stay, with most using Bulgaria as a transit corridor on their way westward.

Bulgaria has erected a barbed-wire fence along its 259km border with Turkey, but with the help of local traffickers many migrants still manage to enter.