The 71 migrants found dead in an abandoned lorry in Austria last week had most likely suffocated soon after they were picked up by a smuggler in Hungary, police said Friday.
Preliminary autospy results indicate that "if you take into account the number of people and lack of oxygen, it's fair to assume that asphyxiation occurred within no time at all," police spokesman Hans Peter Doskozil said.
The final coroner's report was expected to take another five or six weeks, he added.
The victims were believed to have been from the war-torn countries of Syria, Afghanistan and Iraq, Doskozil said.
The 59 men, eight women and four children were discovered piled on top of each other in the back of a refrigerated poultry truck in a motorway layby near the Hungarian border on August 27.
Police made the gruesome discovery after noticed decomposing body fluids dripping from the vehicle.
An Afghan and five Bulgarians have so far been arrested in connection with the case.
One of the suspected drivers, 32-year-old Tsvetan Tsvetanov, denied knowing there had been anyone on board the lorry, during his first appearence in a Bulgarian court on Thursday.
Charges against him include "participation in an organised crime group, contraband trafficking and premeditated manslaughter of 71 people".
Police believe Tsvetanov and the other five -- who were arrested in Hungary -- are low-ranking members of one of the numerous people-trafficking gangs that extract often large amounts of money from migrants to help them reach Europe.
The migrants' deaths led to a security crackdown in Austria and massive tailbacks formed on the border with Hungary earlier this week, as officers inspected vehicles in search of people-smugglers and migrants.