Bulgarian police have arrested four people after they found the bodies of 18 dead migrants from Afghanistan, including that of a child, in an abandoned truck near the capital Sofia, officials said on Friday.

The truck was transporting timber and carrying illegal migrants hidden in compartments, the interior ministry said in a statement, adding it was found near the village of Lokorsko, Reuters reported.

Some of the migrants suffocated, the head of the National Investigative Service, Borislav Sarafov, said.

The migrants crossed the border with neighbouring Turkey illegally and hid in the woods for two days before they were loaded onto the truck near the city of Yambol in southeastern Bulgaria, he told reporters.

Thirty-four migrants, including five children, were rushed to hospitals in Sofia and some were in a critical, but stable, condition, Health Minister Asen Medzhidiev said.

“There has been a lack of oxygen to those who were locked in this truck. They were freezing, wet, they have not eaten for several days,” Medzhidiev told reporters.

One of the four people detained had already been sentenced for human trafficking, said Atanas Ilkov, a senior police official. He said charges would be made once there was enough evidence.

Bulgaria is situated on a route used by migrants from the Middle East and Afghanistan to enter the European Union.

Most do not stay in the country, but look to move on to richer countries in Western Europe, often using elaborate networks of smugglers.

In 2015, three Bulgarian truck drivers were arrested and later charged with the deaths of 71 migrants found dead beside an Austrian motorway.

In December, Bulgaria was blocked from entering the EU’s passport-free Schengen zone by Austria and the Netherlands over security and rule-of-law concerns, but the country will seek to gain entry again this year.