Police in Austria say the bodies of 71 people, thought to be migrants, were found in an abandoned lorry found on a motorway on 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.
It is unclear how they died. Police said they were thought to include a group of Syrian migrants.
Local police say three people, thought to be Bulgarian, have been detained in Hungary.
Police sent to investigate the dumped lorry on the A4 road towards Vienna discovered the decomposing bodies on Thursday morning.
The victims were probably already dead when the vehicle crossed into Austria from Hungary, authorities said.
The vehicle was towed to a customs building with refrigeration facilities in Nickelsdorf where forensic teams worked through the night to examine the bodies.