Austrian forensic scientists have failed to match charred human remains with DNA samples in a Mexican inquiry into missing students, officials say.
Mexican prosecutors said scientists at Innsbruck University were unable to find sufficient DNA in the remains believed to belong to the 43 students.
The laboratory is now offering to carry out a more advanced test on samples not rendered unusable by excessive heat.
The Mexican students disappeared in the south-western city of Iguala on September 26.
The new test would take about three months and it is uncertain whether the examination of the last remaining samples will offer any clues.
It is alleged that the group of trainee teachers was seized by local police before being handed to a drug gang.
Prosecutors say the gang killed them, dumped and incinerated their bodies at a waste site, and scattered the ashes in a local river.
Only one student has been identified from the charred remains.
The forensic scientists at Innsbruck are considered leading experts in identifying damaged remains through DNA testing.
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