Austrian officials now say the bodies of more than 70 people, thought to be migrants, were in an abandoned lorry found on a motorway on Thursday.
They originally estimated that between 20 to 50 people died in the vehicle, found near the Hungarian border.
Police sent to investigate the dumped lorry on the A4 road towards Vienna discovered the decomposing bodies.
The local police chief said it appeared those in the vehicle had been dead for one-and-a-half to two days.
The victims were probably already dead when the vehicle crossed into Austria from Hungary, authorities said. It is unclear how they died.
The vehicle was towed to a customs building with refrigeration facilities in Nickelsdorf where forensic teams worked through the night to examine the bodies.
Austrian police are expected to reveal the exact toll at a news conference at 11:00 (09:00 GMT) on Friday.
The police forces in Austria and Hungary are working together to try and find the driver.
A report in Austrian newspaper Krone (in German) said on Friday that seven people had been arrested in Hungary, but this has not been confirmed.
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 Hungarian prime minister’s chief of staff, Janos Lazar, said on Friday that the lorry was registered to a Romanian citizen in Kecskemet, a city in central Hungary.
Hans Peter Doskozil, police chief in the Burgenland province where the lorry was found, said it was a refrigerated vehicle – not the typical choice for people smugglers, he added.
The lorry, which has Hungarian number plates, is understood to have left Budapest on Wednesday morning.
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 seen parked in lay-by on A4 motorway between Neusiedl and Parndorf
11:30 Thursday: Austrian police open the truck and find bodies
Tens of thousands of migrants from conflict-hit states in the Middle East and Africa have been trying to make their way to Europe in recent months.
A record number of 107,500 migrants crossed the EU’s borders last month.
Some of them pay large sums of money to people smugglers to get them through borders illegally.
Meanwhile, migrants continue to die as they try to reach Europe via the central Mediterranean route.
Hundreds of people are feared to have died after two vessels carrying migrants sank off the coast of Libya on Thursday.
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Source: BBC