BUDAPEST (Reuters) - The driver of a truck in which 71 migrants were found dead beside an Austrian motorway heard their cries for help as their oxygen ran out but followed an order to keep them locked up, Hungarian prosecutors said on Thursday.

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Prosecutors said in a statement they had charged 11 people in connection with the August 2015 deaths, four with murder and all 11 with human trafficking.

The four men participated in the transport of the 71 migrants directly, and the rest were involved in sustaining a network that made hundreds of thousands of euros from human smuggling, said the prosecutors’ office of Bacs-Kiskun county, where the truck began its journey.

Finding clients among the hundreds of thousands of refugees and migrants that crossed the Balkans in 2015, the gang made at least 31 trips across the Hungary-Austria border between February and August.

They often carried people “in closed, dark and airless vans unsuitable for passenger transport, in crowded, inhuman, excruciating conditions,” the prosecution statement said.

In all, the gang moved more than 1,200 refugees to Western Europe and its Afghan boss earned at least 300,000 euros ($330,000).

Recounting details of what they called the “death lorry”, prosecutors said that, around half an hour after departure, the migrants indicated with bangs and cries that they were running out of oxygen. They were heard by the driver of the truck and its reconnaissance vehicle.

“They reported the problem to the Afghan head of the criminal organization who ordered them several times not to open the door of the container hold... (They) complied with the instructions,” the prosecution statement said.

After a three-hour journey, the driver left the truck on the hard shoulder of the A4 highway in Austria and returned to Hungary in the reconnaissance car.

The following day, the gang smuggled another 67 migrants across the border, again using an unventilated, closed refrigerator lorry.

“This time the migrants were able to kick down the door of the container hold, so no one died,” prosecutors said.

They identified the gang leader as a 30-year-old Afghan man, who paid for the operation and organized the transports. A 31-year-old Bulgarian was the head of the human smuggling ring and its two Bulgarian drivers.

Prosecutors are seeking life sentences for the four men, who they charged with murder.

Under Hungary’ judicial system, a judge will now decide whether to proceed to trial on the basis of the prosecution evidence.

Nine of the 11 charged are in custody and the other two are being sought by police.