The bodies of between 20 and 50 refugees have been found in a truck 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.
It has been towed away for forensic examination.
Police said they should be able to give more information on the number of bodies later today.
The grim find came as a summit focusing on migration was taking place in the Austrian capital, Vienna.
Tens of thousands of refugees 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.
The vehicle was there from around 06:00 local time on Thursday morning, Mr Doskozil said.
But Hungarian cameras had identified that the truck was on the Hungarian side of the border, very close to Austria, at 09:00 on Wednesday, he said, so it must have been driven across the border in the intervening time.
The decomposing state of the bodies would suggest that they had been dead for one-and-a-half to two days, the police chief added, suggesting that the victims must have already been dead when the lorry crossed into Austria.
On Thursday afternoon the truck was towed away to undergo forensic examinations in Nickelsdorf.
Police will give a further news conference later today, when they should be able to say how many people had died.
They do not yet know whether women and children are among the victims, or how they died.
The truck bears the logo of a Slovakian poultry company, Hyza, which said in a statement that the vehicle no longer belonged to the firm - but the new owners had not removed the branding.
Hungarian police are working with Austrian police on the investigation, a spokesman for the Hungarian prime minister said.
Hungary had been informed that the driver was Romanian, the spokesman said.