By Michael O'Boyle and Simon Gardner
MEXICO CITY (Reuters) - Tests have identified the remains of one of 43 trainee teachers abducted 10 weeks ago in southwestern Mexico, a source close to the investigation said on Saturday, apparently confirming a feared massacre the government has blamed on drug gang henchmen.
The brazen attack has sent shockwaves through Mexico, shining a spotlight on the nexus between impunity, corruption and drug gangs that has blighted Latin America's No.2 economy for years.
President Enrique Pena Nieto is facing his deepest crisis over his government's handling of the probe. The government says the students were murdered and incinerated, their remains then tipped into a garbage dump and a river.
Thousands marched in Mexico City on Saturday evening, chanting "Out Pena" and "They were taken alive, we want them alive."
"If these disappearances and executions have grown over the years, it is because the authorities have permitted this to happen," said Itzel Silva, a 38-year-old human rights lawyer. "People are fed up with impunity."
The source close to the investigation said that Argentine forensic experts working in Mexico had told the family of the identified trainee teacher.
"Today Argentine specialists confirmed to my father that the remains were me," said a post on the Facebook page of the radical leftist college the students attended, written in the voice of Alexander Mora, one of the missing trainee teachers.
Mexico's attorney general's office said it could not confirm the positive identification. Attorney General Jesus Murillo scheduled a news conference for Sunday.
"Now we know that these are our children," Nardo Flores, the father of another of the missing students, told Reuters. "All we can hope now is that justice is done, and that the government punishes those responsible."
The apparent massacre has spurred widespread and sometimes violent protests throughout Mexico, where more than 100,000 people have been killed in gang-related violence since 2007.
The students were abducted by corrupt police on Sept. 26 during a demonstration in the town of Iguala in the southwestern state of Guerrero, and handed over to a drug gang, the attorney general has said.
The case has piled major pressure on Pena Nieto, who is also grappling with a weaker-than-expected economy and the fallout from a conflict of interest scandal, overshadowing his efforts to focus attention on a major economic reform drive.
(With reporting by Lizbeth Diaz; Editing by Paul Simao and Grant McCool)