Austria says the lorry found abandoned near the Hungarian border on Thursday contained the bodies of 71 people. They are thought to have been refugees from Syria.
The vast majority, 59 in total, were men. There were also eight women and four children including a baby girl.
The authorities believe they suffocated.
A number of people have been detained in Hungary, according to police from both countries.
Austrian officials gave more details at a news conference on Friday morning.
“We think that one perpetrator is a Bulgarian citizen, originally from Lebanon. He is the owner of the vehicle. Of the two other perpetrators, there is another Bulgarian citizen and one person with a Hungarian identity card,” said Hans Peter Dokozil, police chief for the Burgenland region.
Hungarian police said in a statement on their website that four people had been arrested in relation to the tragedy: three Bulgarians and one Afghan.
The vehicle with a Hungarian licence plate was found by a motorway patrol on the A4, one of the main routes to Vienna. It had previously been spotted near Budapest.
The refrigerated air-tight truck bore the logo of a Slovakian poultry company, Hyza, which reportedly says the lorry had been sold.
It’s thought those inside had been dead for up to two days; the bodies were badly decomposed.
Separately, police in Hungary said on Friday that 10 Syrians were injured overnight when their van overturned on a motorway heading for Budapest. They are thought to have been among a group of at least 18.
Thursday’s grisly discovery coincided with a key summit on migration in Vienna, where Angela Merkel began talks with six Balkan nations and senior EU officials.
The German Chancellor said the number of refugees in Europe had reached the highest level since the Second World War.
The lorry tragedy focuses attention on Austria’s southeast and Hungary, which many migrants and refugees pass through as they try to get to Vienna or on to Germany. It has also brought calls for the Balkans to be given more attention as part of an EU-wide solution to the migration crisis.