Four human traffickers were jailed for life in a Hungarian court on Thursday for the deaths of 71 migrants.

The decomposing bodies of the migrants were found in a truck that was abandoned on the side of an Austrian motorway in 2015.

The sentence was the final ruling of an earlier verdict that handed out 25 years in prison to the four defendants last year.

The Afghan ringleader of the gang and three Bulgarian accomplices were found guilty of manslaughter after 59 men, eight women and four children suffocated in the back of a locked truck on a Hungarian motorway.

The cold-room truck was purchased in Hungary prior to the journey and the defendants had knowledge that the back of the truck was air-tight before it was used, the court heard.

The migrants realised quickly the danger of suffocation in the overcrowded truck, so they banged on the doors to alert the driver, an investigative judge said earlier this year.

Despite their screams, the traffickers refused to open the doors.

"The suffering in that truck is beyond imagination, as is the utmost indifference the perpetrators showed toward the deaths of these 71 people," said presiding judge Erik Mezolaki. "Therefore for those who played a core part, there is no doubt that the only punishment commensurate with their crime is the maximum one, which is life in jail without parole."

Mezolaki said that Samsooryamal, Afghan ringleader, had given a direct order to the truck's driver not to open the doors or even give the passengers water. "If they die, leave them somewhere in Germany," he said.

During the drive, Samsooryamal was in contact with the three Bulgarians, Metodi Georgiev, Vencislav Todorov, and Ivaylo Stoyanov, one of whom drove the truck, while the other two followed in cars.

"They stopped first at about 6 am, but the victims made so much noise that he (Georgiev) instructed them (Todorov and Stoyanov) to keep going," Mezolaki said at the trial in Szeged.

"At 7 am, the instruction was to see if they were still screaming so hard, and if not, stop and get lost. By that time, 68 of the victims had died," he added.

All of the passengers suffocated within two hours of departure.

According to Mezolaki, the defendants had many opportunities to alleviate the passengers' suffering. The traffickers had bought a metal cutter but decided not to use it to open the truck.

The truck containing the 71 bodies was dumped near Pandorf immediately after crossing into Austria, with the rear doors sealed shut with wire.

The two ringleaders were previously under investigation by Hungarian authorities, who were aware of their smuggling activities but the traffickers were allowed to continue operating.

A total of 14 suspects were convicted in relation to the crime, who between them smuggled over 1,200 migrants to Western countries.

The four men charged Thursday typically smuggled 20-25 people per trip.

At the peak of the migrant crisis in 2015, the traffickers took advantage of the sheer volume of people attempting to reach Europe and overcrowded the truck.

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