Police in Austria say the bodies of 71 people, thought to be migrants, were found in an abandoned lorry found on a motorwayon Thursday.
The bodies of 59 men, 8 women and four children are thought to have been dead for one-and-a-half to two days.
Police said the victims appeared to be migrants from Syria and probably died after suffocating in the vehicle.
Three people, thought to be Bulgarian, have been detained in Hungary. They are believed to have driven the lorry.
Police sent to investigate the dumped lorry on the A4 motorway towards Vienna discovered the decomposing bodies on Thursday morning.
The vehicle was towed to a customs building with refrigeration facilities where forensic teams worked through the night to examine the bodies.
The victims are thought to have been dead when the vehicle crossed into Austria from Hungary.
The local police chief said a travel document found on the vehicle suggested that the group were Syrian migrants.
"We found a Syrian travel document, so our preliminary assumption is of course that they were refugees, possibly a group of Syrian refugees," Hans Peter Doskozil, Burgenland province police chief, told reporters.
Among the victims was a girl aged between one and two, and a boy aged between eight and nine years old.
He said the three people being held in Hungary would "lead to the perpetrators", adding that there was "an indication we are talking about a Bulgarian-Hungarian human trafficking operation".
The vehicle had the branding of a Slovakian poultry company, Hyza, on it but the firm said it no longer owned the vehicle.
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.
Mr Doskozil said it was a refrigerated vehicle and was not the typical choice for people smugglers.
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.