- Mexico's former attorney general has been arrested in connection with the disappearance of 43 students in 2014.
Jesús Murillo Karam, who led an inquiry into the atrocity, has been charged with forced disappearance, torture and the obstruction of justice.
The students vanished while travelling by bus through the city of Iguala on their way to a protest in Mexico City.
Other than bone fragments recovered from three of them, nothing is known of their fate.
Municipal police opened fire on buses carrying the students on the evening of 26 September 2014 - but what happened next is disputed.
Their mysterious disappearance sent shockwaves around the world and triggered widespread protests in Mexico against impunity and state complicity in organised crime.
Jesus Murillo Karam, who was arrested on Friday, led a controversial 2015 investigation into what happened to the students that blamed members of a cartel who were accused of killing them and burning their remains.
His findings, which were endorsed by then President Enrique Peña Nieto, were criticised by independent experts and relatives of the missing students for errors and for not apportioning any blame to the armed forces.
In a tweet, Mr Karam's Institutional Revolutionary Party (PRI), which is no longer in power, accused those behind Friday's arrest of being politically motivated.
He is the highest-profile government official to be arrested in connection with the disappearance of the students so far.
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Investigators believe the students were detained by corrupt police officials and then handed over to a drug cartel who mistook them for members of a rival gang before killing them.
On Thursday, a truth commission established by current President Andrés Manuel López Obrador accused soldiers of bearing some responsibility for the massacre - at least through negligence, if not directly.
"Their actions, omissions or participation allowed the disappearance and execution of the students," said Alejandro Encinas, who is the commission's head as well as Mexico's deputy interior minister, according to AFP news agency.
But the full role of military personnel required further investigation, he added.
Earlier this year President López Obrador revealed that navy members were being investigated for allegedly tampering with evidence, including at the garbage dump where human remains were found.
On Friday, he called for any soldiers or officials involved in the disappearance to face justice.
"Publicising this atrocious, inhuman situation and at the same time punishing those responsible helps to prevent these deplorable events ever happening again," the Mexican leader said, according to AFP news agency.