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An abandoned truck "full of bodies" was found on the side of a highway in eastern Austria on Thursday.
Police said the dead were thought to be refugees.
Gerald Pangl, a police spokesman in the Austrian state of Burgenland, told NBC News that the death toll was unclear.
Hans Peter Doskozil, the head of the Burgenland Police, added: "We cannot say exactly how many there are. We could imagine that maybe 20 people have died, but it could also be 40 or 50 ... We think that these are refugees."
The Krone newspaper reported that initial indications were that they suffocated.
According to Pangl, the vehicle's cab was unlocked and there were no signs of its driver. He added that the truck had Hungarian license plates.
Pangl said colleagues described the vehicle as being "full of bodies."
The 7.5-ton truck was found on a highway near the town of Neusiedl am See, around 30 miles from Vienna and 10 miles from the Hungarian and Slovakian borders. The road forms part of a major artery linking Eastern Europe to France and Germany.
Temperatures in Vienna were 80F Wednesday and 90F Thursday, according to data from Accuweather and the BBC. Temperatures in the nearby Slovakian city of Bratislava hit 85F.
The war in Syria has contributed to record numbers of migrants attempting to cross into Europe from Africa and the Middle East this year, often taking perilous sea crossings and stowing away in trucks.
Images of the truck taken by British network Channel 4's Lindsey Hilsum show it was branded with the logo of Slovakian poultry producer Hyza.
Hyza spokesman Nicholas Šranko told NBC News that the truck found in Austria was one of 21 older vehicles it had sold on to other Slovakian companies in 2013 and 2014.
"We are really sorry about the tragedy that happened in Austria — today is a really black day," he said. "Unfortunately we did not remove the branding before we sold the vehicle, but we will be sure to do this in the future."
He said the firm was now working with Slovakian police to track who purchased the vehicle.
A spokeswoman for the office of Hungarian Prime Minister János Lázár told NBC News that the vehicle had been registered to a Romanian citizen in the Hungarian city of Kecskemét.