MEXICO CITY (AFP) - The authorities in Mexico can now say with "legal certainty" that the 43 students who went missing in September were murdered by hitmen working for a drug gang, the justice minister said Tuesday.

The disappearance of the men - all aspiring teachers attending classes at a training college in the southern state of Guerrero - sparked nationwide protests and a crisis for the government of President Enrique Pena Nieto.

Officials said the students vanished after gang-linked police attacked their buses in the city of Iguala, allegedly under orders from the mayor and his wife in a night of terror that left six other people dead.

The police then delivered the young men to members of the Guerreros Unidos drug gang, who told investigators they took them in two trucks to a landfill, killed them, burned their bodies and dumped them in a river.

The investigation "gives us the legal certainty that the student teachers were killed in the circumstances that have been described," Justice Minister Jesus Murillo Karam told a press conference.

Witness and expert testimony "have allowed us to... come to the conclusion beyond a doubt that the students were abducted and killed, before being incinerated and thrown into the San Juan river, in that order," he said.

Until now, authorities had still officially considered the students to be missing.

Relatives of the victims, who marched on Monday with several thousand people in Mexico City to mark the four-month mark since their disappearance, have refused to accept the official explanation of events.

For now, only one of the students has been positively identified from charred remains, which leaves little hope of finding the 42 others.