Nearly seven years after 43 students went missing in Mexico's Guerrero state, the remains of a third victim have been identified beyond doubt, prosecutors said on Tuesday.

Using DNA testing, forensic doctors at the University of Innsbruck, Austria, identified a lumbar vertebra as belonging to Jhosivani Guerrero, said Omar Gomez Trejo, head of the special unit of the Mexican Attorney General's Office that is reinvestigating the case.

On September 26, 2014, 43 young men studying at a rural teacher training college in Ayotzinapa went missing in the city of Iguala while travelling on buses they had stolen.

They were pursued by police and allegedly handed over to the crime syndicate Guerreros Unidos, for reasons that are not known.

An earlier investigation said their bodies had been burned at a rubbish dump, but that version of events was later discredited.

President Andres Manuel Lopez Obrador's government relaunched the investigation after the previous one was plagued with irregularities and more than half of the over 140 suspects were released.

Dozens of suspects have so far been arrested in connection with the case, including Iguala's police chief and the city's mayor at the time. To date, no one has been convicted.

Two other students have been previously identified from human remains uncovered by forensic teams.

Last July, a student was identified from a bone fragment found in a gorge at 20 kilometres from the site of the students' disappearance, the same site where Guerrero's vertebra was found.