Atleast 20 - and possibly up to 50 - badly decomposed bodies of migrants have been recovered from a truck parked on the shoulder of the main highway from Budapest to Vienna, said Austrian police.
The shocking find came as Austria hosted a summit in Vienna on Europe's refugee crisis for Western Balkan nations, which have been overwhelmed this year by the tens of thousands of migrants trying to get into Europe via their territory.
Police ordered reporters at the scene 25 miles (40 kilometers) southeast of Vienna to move away from the vehicle, a white refrigeration truck with pictures of chicken on it. The truck, with all the bodies still inside, was later taken away to a secure location so forensic experts could better examine it.
The state of the bodies on a hot summer day made establishing the identities and even the exact number of dead migrants difficult, but the total number could rise to 50, said Hans Peter Doskozil, chief of the Burgenland police.
Police spokesman Helmut Marban said police stopped shortly before noon Thursday thinking that the parked truck had some mechanical trouble. Then they "saw blood dripping" from the vehicle and "noticed the smell of dead bodies," he said.
The truck was apparently abandoned Wednesday and its back door was left open, Doskozil said. It had Hungarian license plates but the writing on its side and back was in Slovak. The state of the bodies suggested the migrants could have been dead for several days.
Police said the investigation could last for days. They declined to give further information on the victims' possible identities, whether children were among them, how the migrants may have died or other details.
Government officials and rights groups condemned the traffickers.
"Human smugglers are criminals," said Austrian Foreign Minister Johanna Mikl-Leitner. "Those who still think that they are gentle helpers of refugees are beyond saving."
Speaking on the sidelines of the refugee crisis conference, Austrian Chancellor Werner Faymann said the deadly tragedy showed how critical it was for nations to work together on solutions to the influx of migrants.
"Today refugees lost the lives they had tried to save by escaping, but lost them in the hand of traffickers," he told reporters.
German Chancellor Angela Merkel, who was also at the summit, said she was "shaken by the awful news that up to 50 people lost their lives on their way to look for more security."
"This reminds us that we in Europe need to tackle the problem quickly and find solutions in the spirit of solidarity," she said.
The truck apparently used to belong to the Slovak chicken meat company Hyza, part of the Agrofert Holding, which is owned by the Czech Finance Minister Andrej Babis.
Agrofert Holding, in a statement, said they had sold the truck in 2014. The new owners did not remove the truck's logos as required and Hyza has nothing to do with the truck now, the company said.
On one side of the truck was the slogan "Honest chicken," while writing on the back read "I taste so good because they feed me so well."
The Hungarian government said the truck's license number plates were registered by a Romanian citizen in the central city of Kecskemet.
Migrants fleeing war and poverty from the Middle East, Africa and Asia are flocking to Europe by the hundreds of thousands this year.
Many follow the Balkans route, from Turkey to Greece by sea, up north to Macedonia by bus or foot, by train through Serbia and then walking the last few miles into EU member Hungary. That avoids the more dangerous Mediterranean Sea route from North Africa to Italy, where the bodies of 51 migrants were found Wednesday in the hull of a smugglers' boat rescued off Libya's northern coast.
Once inside, the 28-nation EU, most migrants seek to reach richer nations such as Germany, The Netherlands, Austria or Sweden.
Hungarian police said they detained 3,241 migrants on Wednesday, over 700 more than a day earlier and the highest number so far this year.
The Hungarian government is quickly finishing a razor-wire border fence to keep the migrants from crossing from Serbia.
Amnesty International alleged that EU indecisiveness was partly to blame for the latest migrant tragedy.
"People dying in their dozens - whether crammed into a truck or a ship - en route to seek safety or better lives is a tragic indictment of Europe's failures to provide alternative routes," said a statement from the rights group.
"Europe has to step up and provide protection to more, share responsibility better and show solidarity to other countries and to those most in need."