Sixty of the 71 victims were men. Eight women died, as did three children, ages 2, 3 and 8.
"We are talking about human trafficking, homicide, even murder," said Johann Fuchs, state prosecutor of Eisenstadt.
News of the deaths comes as Europe is struggling to deal with a record swell of migrants and refugees traveling by land and sea to find a better life or escape conflict at home in the Middle East and Africa.
United Nations officials have called on European officials to improve their efforts to resolve the growing humanitarian crisis, including dealing with human traffickers preying on refugees.
"This tragedy underscores the ruthlessness of people smugglers who have expanded their business from the Mediterranean Sea to the highways of Europe," said Melissa Fleming, a spokeswoman for the office of the United Nations High Commissioner for Refugees. "It shows they have no regard for human life and are only after profit."
The truck was found Thursday, abandoned on the side of the A4 highway, which links Budapest in Hungary to the Austrian capital, Vienna. When police opened it, they found the bodies decomposing in the heat.
They found a Syrian travel document inside the truck.
"We must assume now that these are refugees," said Hans Peter Doskozil, police director of Austria`s Burgenland region. "In concrete terms, it is possible this is a Syrian refugee group."
Hungarian police said they had arrested three Bulgarian citizens and an Afghan citizen in connection with the truck.
"We are likely looking at a Bulgarian-Hungarian people smuggling ring," Fuchs said.
About 3,000 trucks pass through that area of the highway daily, and checking each one of them is almost impossible, authorities said. Most human smugglers use smaller cars, making the use of trucks unusual.
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