Migrant crisis: Austrian police work to recover bodies from lorry
Forensic teams have worked through the night to establish how many people died inside a lorry found abandoned on a motorway near the Hungarian border, Austrian police say.
Police estimate 20 to 50 people, almost certainly migrants, were in the lorry.
The victims were probably already dead when the vehicle crossed into Austria, authorities said.
The local police chief said he hoped to give more information on the number of bodies on Friday morning.
Hans Peter Doskozil said the lorry had been towed to a former veterinary border facility with refrigeration facilities in Nickelsdorf, where the bodies were to be removed.
The corpses would be transported to Vienna for further examinations and to establish the cause of death.
Tens of thousands of migrants from conflict-hit states in the Middle East and Africa have been trying to make their way to Europe.
Some of them pay large sums of money to people smugglers to get them through borders illegally.
Austria's Chancellor Werner Faymann said the tragedy showed once again "how necessary it is to save lives by combating criminals and people traffickers".
On Thursday morning Austrian police in the state of Burgenland were alerted to the presence of the vehicle in a lay-by off the A4 motorway, between Neusiedl and Parndorf, heading towards Vienna.
The officers first on the scene were able to open the rear doors and were faced with decomposing bodies, Mr Doskozil said.
The state of decomposition would suggest that they had been dead for one and a half to two days, he said.
The vehicle bears the logo of a Slovakian poultry company, Hyza, which said it no longer owned the vehicle - but the buyers had not removed the branding.
The police chief said it was a refrigerated vehicle - not the typical choice for people smugglers, he added.
Mr Doskozil said his officers were working with colleagues from Hungary on the investigation.
The lorry has Hungarian number plates.
Burgenland police will give a further news conference at 11:00 local time (09:00 GMT) on Friday.
Early Wednesday: Police believe the truck set off from south-east of Budapest
09:00 Wednesday: Truck recorded on cameras at Hegyeshalom on the Hungarian side of the border with Austria
05:00 or 06:00 Thursday: Truck was already parked in the lay-by at this time
11:30 Thursday: Austrian police open the truck and find bodies
Source: Austrian police press conference
In Vienna, Serbia and Macedonia have told a summit that the 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.
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 so far 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.
There were two more shipwrecks off Libya on Thursday, with hundreds of migrants feared dead.