The bodies of between 20 and 50 migrants have been found in a lorry abandoned in a motorway lay-by near the Austrian border with Hungary, authorities say.
Austrian police say they think the people inside were already dead when the truck crossed into Austria.
The lorry has been towed away for forensic examination.
Police say they should be able to give more information on the number of bodies on Friday morning.
The grim find comes as a summit focusing on migration takes place in the Austrian capital, Vienna.
Tens of thousands of migrants from conflict-hit states in the Middle East and Africa have been trying to make their way to Europe.
Austria’s Chancellor Werner Faymann said the tragedy showed once again “how necessary it is to save lives by combating criminals and people traffickers”.
The vehicle – a refrigerated lorry with Hungarian licence plates – was found parked off the A4 motorway between Parndorf and Neusiedl am See, according to Burgenland police chief Hans Peter Doskozil.
