Over a month after 43 students disappeared in southern Mexico, demonstrators are still taking to the streets, determined to find out what became of them.

Initial testimony suggested the students had a history of conflict with the local mayor, with police accused of handing them over to gangsters who killed them.

Investigators have arrested more than 50 people, including police officers and suspected gang members

Tomas Zeron, Federal Chief of Criminal Investigations, said the inquiry was focussing on statements indicating that the students were indeed delivered by officers to a criminal gang.

As a search for them continues, the local mayor in the southern city of Iguala and his wife are still on the run after arrest warrants were issued for them.

Mass graves have been uncovered but none of the bodies found appear to belong to any of the missing students whose relatives have met Mexico’s President Enrique Pena Nieto. They dismissed his efforts to find their loved ones and said their patience was running out.

The disappearances have become arguably the sternest challenge yet to face Pena Nieto, who took office two years ago vowing to restore order in Mexico, where close to 100,000 people have died in violence linked to organised crime since 2007.