Hungarian police said yesterday three of the suspects held over the latest grim tragedy involving migrants seeking refuge in Europe were Bulgarians and one was Afghan.
Thursday's grisly discovery of the truck on a motorway near the Slovakia and Hungary borders - a rare occurrence on land, with most migrant deaths taking place at sea - sent shockwaves through Europe.
The news of the arrests came as Libyan rescue workers recovered 76 bodies from yet another capsized boat in the Mediterranean crammed with people fleeing conflict and hardship in the Middle East and Africa.
Many of the victims were of Arab or African origin, a Pakistani teenager who survived by clinging to wreckage for nine hours told AFP.
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Austrian police said the truck victims were likely fleeing the war in Syria and included a toddler and three young boys.
"Among these 71 people, there were 59 men, eight women and four children including a young girl one or two years old and three boys aged eight, nine or 10," police spokesman Hans Peter Doskozil told a news conference.
Doskozil said those arrested included the owner of the vehicle and two drivers, and were likely "low-ranking members... Of a Bulgarian-Hungarian human-trafficking gang".
A court today is to decide whether they can be detained beyond an initial 72-hour period.
Austria's public prosecutor Johann Fuchs said he would likely seek to have the suspects extradited.
Yesterday evening, Hungarians held a vigil for the victims outside Budapest's main train station, where thousands of migrants have been sheltering for weeks.
Austrian motorway maintenance workers alerted police after noticing "decomposing body fluids" dripping from the truck, Doskozil said.
Television images showed flies buzzing around the back of the vehicle in the baking sun.