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.
The vehicle was there from around 06:00 local time (04:00 GMT) 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 at 11:00 on Friday, when they should be able to say how many people have died.
They do not yet know who the victims were 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.
At the scene: Bethany Bell, BBC News
The lorry in which dozens of people died was sitting in a lay-by on the main motorway between Vienna and the Hungarian and Slovakian borders.
As we drove past, I saw police in white forensic suits combing the area for evidence.
It is hot. The smell of so many decomposing bodies close up must be difficult to take.
In Vienna, Serbia and Macedonia have told the summit on migration that EU must come up with an action plan to respond to the influx of migrants into Europe.
Austria has complained that the EU has failed to address the problem of people entering via the Western Balkans.
German Chancellor Angela Merkel said Thursday's find "reminds us to tackle the issue of migration with European spirit and find solutions".
A record number of 107,500 migrants crossed the EU's borders last month and on Wednesday police counted more than 3,000 crossing into Serbia.
Many make their way north from Greece. The International Organization for Migration said on Thursday that an estimated 209,457 migrants had entered Greece between 1 January and 25 August this year, nearly five times the number that arrived in the whole of 2014.
Meanwhile migrants are continuing to die as they try to reach Europe via the central Mediterranean route. The bodies of at least 51 people were found on Wednesday in the hold of a stricken ship off the coast of Libya.