MEXICO CITY (Reuters) - Mexico said on Monday it had arrested a suspected drug gang member regarded as key figure in the kidnapping and massacre of 43 student teachers 3 1/2 years ago.

Erick "N", suspected of being linked to the disappearance of 43 students of the Ayotzinapa teacher training college 'Raul Isidro Burgos', is pictured after being detained, in this undated handout released to Reuters by the PGR Attorney General's Office, March 12, 2018. PGR Attorney General's Office/Handout via REUTERS

The atrocity had plunged President Enrique Pena Nieto’s government into one of its worst crises, as doubts swirled around the conduct of the investigation into the case.

The attorney general’s office said it had arrested Erick N, “a probable member of a criminal organization” operating in the violent southwestern state of Guerrero.

The government had put a 1.5 million pesos ($81,000) bounty on Erick N.’s head because of his alleged involvement in the murder of the student teachers, who went missing in September 2014.

An initial investigation had found that the victims were abducted by corrupt police who handed them over to members of a local drug cartel, who then killed them, incinerated their bodies at a trash dump and threw the ashes into a river.

However, the official account has been widely questioned by local and international human rights experts.

During his six-year-term, which will come to an end in December, the murder rate has climbed, with more than 25,000 committed across the drug-ravaged country in 2017, the highest since modern records began.

Earlier this month, the prosecutor in charge of the case said Mexican authorities were preparing to arrest dozens of people implicated in the killing of the student teachers.

Last week, U.N. High Commissioner for Human Rights Zeid Ra’ad al-Hussein said he would soon issue a report “on elements of the investigation” in the case.

($1 = 18.5730 Mexican pesos)