Hungarian police said Friday they arrested four men in connection with the scores of migrants found dead Thursday in a truck in Austria, a discovery that deepened the debate across Europe about how to accommodate or stem the biggest inflow of migrants in its post-War history.

The men, three Bulgarians and one Afghan, are suspected of being part of a clandestine migrant smuggling network, the Hungarian police said.

Austrian police said earlier Friday that the bodies of 71 migrants had been removed froman abandoned delivery truck on a highway some 20 miles from the Austrian-Hungarian border. The migrants were thought to have suffocated or died of thirst, the police said.

The gruesome discovery in the heart of the continent shocked European Union countries, which have struggled to address the crisis sparked by the arrival of hundreds of thousands of migrants from the Middle East, Africa and the western Balkans since the start of 2014.

As they scramble to house and process the newcomers, national governments, often under pressure from hostile or fearful electorates, have also sought to stem the influx, beefing up border controls and debating cuts to the handouts some argue have acted as a magnet for economic migrants with unfounded asylum claims.

An EU initiative to distribute migrants, now concentrated in a handful of countries, more broadly across the region has faced stiff resistance in several key countries.

The bodies of 59 men, eight women and four children were discovered in the refrigerator truck on Thursday, Austrian regional police chief Hans Peter Doskozil told a news conference Friday. Based on travel documents found in the truck, the police assume that most of the victims were refugees from Syria.

Police and forensic medical examiners in Vienna are working to ascertain the migrants’ identities, as well as the time and cause of their deaths. Investigators believe the migrants had been dead for at least one-and-a-half to two days when they were discovered.

The highway where the truck was discovered is one of the main migration routes from eastern to western Europe, Austrian police said Thursday.

Many migrants are now traveling from Turkey to Greece and then across the Balkans to Hungary, a course considered less risky than the often deadly sea route across the Mediterranean.

Around 3,000 people a day are currently being moved through the Balkans, the United Nations High Commissioner for Refugees estimates. The route has been used by roughly 10 times as many migrants so far this year as over the same period in 2014, according to the EU border agency Frontex.

Smuggling has become a lucrative business for criminal rings in Europe as more migrants have converged on the continent and tougher controls at the EU’s borders have made it more difficult to enter the region without outside help. The Austrian police estimate migrants pay between $3,370 and $5,000 for the trip from Syria, Afghanistan or Iraq.

Prosecutors in Italy, which has faced a large inflow of migrants coming by sea, say one of the main challenges is catching the masterminds behind smuggling rings—small groups that operate through a web of contacts mainly in Europe, Libya, Turkey and the countries of origin of the migrants and refugees.

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