The bodies of more than 70 migrants were in the lorry that was found abandoned in Austria on Thursday, police have said.
When police were first alerted to the 7.5 ton truck, the corpses were said to be so decomposed it was difficult to put an estimate on the number inside.
It was originally thought there were between 20 and 50.
Investigators have been examining the lorry in detail in order to work out how many were inside.
It is thought the migrants may have been dead when the vehicle was driven across the border from Hungary to Austria.
Forensic specialists clad in white protective suits and yellow rubber boots have been seen wheeling body bags away from the refrigerated customs building in the village of Nickelsdorf where the vehicle was taken.
It was found shortly before noon local time (11am UK time) by an Austrian highway patrol which had spotted the vehicle on the edge of the A4 highway, close to Parndorf.
The spot is about 50km (28 miles) southeast of the capital Vienna and about 25km (16 miles) from the Hungarian and Slovak borders.
Austrian newspaper Krone said that early investigations have found the truck was in Budapest in the early hours of Wednesday and was seen at the border at around 9am.
The truck was then seen in Austria at around 5am on Thursday and when it failed to move for some time, witnesses called the police.
When officers arrived they found the back door slightly ajar and inside were the partially decomposed remains of the migrants.
"When they checked they found it had no driver and blood was dripping out of the vehicle and there was a smell of dead bodies," Helmut Marban, press officer for Burgenland police, said.
It is thought the people were trapped in the trailer and suffocated after being unable to get out but police said it was too early to confirm the cause of death.
At a press conference in Eisenstadt, Interior Minister Johanna Mikl-Leitner said it demonstrated "the despicable methods used by Mafia traffickers in all their ugliness in Austria".
Police suspect those responsible have already fled the country.
They said the truck had Hungarian licence plates and the logo of a Slovakian meat product company on the side.
The company told the Krone newspaper it had sold 13 vehicles in 2014.
A spokesman said he understood that the buyer had then resold at least one of them on in Hungary.
German Chancellor Angela Merkel said the discovery had shaken European leaders discussing the migrant crisis at a Balkans summit.
She said: "We are of course all shaken by the appalling news. This reminds us that we must tackle quickly the issue of immigration and in a European spirit - that means in a spirit of solidarity - and find solutions."
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