Chilpancingo, February 6 (RHC-EFE) -- Some 20,000 people occupied a stretch of one of Mexico's busiest highways on Thursday to demand the safe return of 43 kidnapped students who the government says are dead.

Led by the parents of the students who were abducted September 26th, the group set out at midday from Chilpancingo, capital of the southern state of Guerrero, to march down the Autopista del Sol in the direction of the Pacific resort of Acapulco.

The spokesman for the students' families, Felipe de la Cruz, said they will not relent until they get a convincing explanation of what happened to their loved ones. The protesters also called for a withdrawal of federal security forces from Guerrero, accusing those forces of harassing the supporters of the students.

Authorities have established that the 43 students from Ayotzinapa teachers college were seized by municipal police in Iguala, Guerrero, and handed over to members of the Guerreros Unidos drug cartel, who murdered the youths, burned the bodies and dumped the bones into the San Juan River, Mexican Attorney General Jesus Murillo Karam said last week.

Families of the students remain unwilling to accept that version of events and are demanding to know why soldiers of the Iguala-based 27th Infantry Battalion who witnessed the police attack did not intervene.

In December, respected newsweekly Proceso published a story drawing on a confidential Guerrero state government document that points to Mexico's Federal Police as the perpetrators of the slaughter of the 43 students. And even before the Proceso report, a group of scientists at the National Autonomous University of Mexico said that Murillo Karam's account of the burning of the students' bodies "has no support in facts or in physical, chemical or natural phenomena."

A delegation representing the students' families traveled this week to Geneva to denounce the Mexican government before the U.N. Committee on Enforced Disappearances.