Mexico missing students: Knowns and unknowns
On the evening of 26 September 2014, a group of 43 Mexican students disappeared in south-western Guerrero state. Their relatives have been searching for them ever since. There have been three investigations into the events that night and a fourth is still under way. Their findings have differed widely. Below we look at what is known and what is not known about the missing 43.
Who are they?
The 43 studied at an all-male teacher training college in the town of Ayotzinapa. The college has a history of left-wing activism and the students regularly took part in protests.
What were they doing on the day of their disappearance?
The 43 were part of a larger group of teacher trainees from Ayotzinapa who travelled to the nearby town of Iguala to protest against what they saw as discriminatory hiring practices for teachers.
They also wanted to raise funds for a trip they were planning to Mexico City to mark the anniversary of the 1968 Tlatelolco Massacre, when a large number of students were killed by the security forces in the capital.
One of the group said they were planning on commandeering a number of buses for their trip, although he insisted they would convince the drivers to take them rather than force them.
What happened that night?
As they were travelling back from Iguala to Ayotzinapa, they were confronted by municipal police, who opened fire on the buses they were travelling in.
The officers maintain they did so because the buses had been hijacked, while the surviving students say the drivers had agreed to give them a lift.
Police also mistakenly fired on a bus carrying a local football team, killing its driver and one of the players on board. A woman travelling in a nearby taxi was also killed by a bullet.
Three students were also killed, two of them shot dead, while the body of the third was found mutilated the next morning near the scene of the shooting.
What about the missing 43?
A group of 43 students has been missing since the clash between the municipal police and the students.
According to the official government report, they were seized by municipal officers and taken to the police station in the nearby town of Cocula.
There, they were handed over by corrupt police officers to members of a local drugs gangs, Guerreros Unidos (United Warriors), the report says.
The gangs then took them to a local rubbish dump, where they killed them and burned their bodies, the official report continues.
They dumped their bones and ashes in a nearby stream, it concludes.
Have any of their remains been found?
Independent forensic experts have matched charred bone fragments reportedly found at a rubbish dump near Iguala to Alexander Mora, one of the 43 missing students.
They also say there is a high probability another set of remains could belong to Jhosivani Guerrero de la Cruz, another of the students.
However, experts from the Inter-American Commission of Human Rights said in September 2015 that the chain of evidence was broken and that they could not be sure the bone fragments had been found at the dump.
Another independent investigation by Argentine forensic experts also concluded on 9 February 2016 that there was no biological or physical evidence to indicate that the bodies of the 43 students were burned at the rubbish dump.
If they were not killed in Cocula, where are they?
Some of the relatives of the missing students are clinging on to hope that they may still be alive, despite the Attorney General at the time declaring them dead.
They point to the fact that so far, only the remains of Alexander Mora have been positively identified.
They suspect that the students may have been taken to the local army barracks and have demanded access to the barracks, which they have been denied.
Why do the relatives think the army may have played a role?
Investigators from the Inter-American Commission of Human Rights found that the army was alerted to the clash between the police and the students on the night of the events.
They said that the 27th Battallion, stationed in Iguala, monitored the movements of the students from the time that they were alerted to the clashes.
The government has refused to let the soldiers be questioned by anyone but government prosecutors, which has raised suspicions among the relatives that there is a cover-up.
Why did the students become a target in the first place?
There have been many theories as to why the students may have been targeted.
Their left-wing activism and protests in Iguala had angered the mayor of Iguala at the time, Jose Luis Abarca, even before the night of the clash.
On the day the students came to Iguala, Mr Abarca's wife, Maria de los Angeles Pineda, was giving a speech at the town square, and there was speculation police were sent to stop the students from disrupting her event.
The couple fled the town after the students' disappearance and were later arrested in Mexico City. They are currently in prison awaiting trial on charges of links with organised crime.
They allege they were tortured by the security forces and forced into giving false evidence in the case of the missing students.
The mayor of Cocula, where corrupt officers reportedly handed the students over to the Guerreros Unidos gang, is also facing charges of links with organised crime.
Dozens of police officers and Guerreros Unidos gang members are also under arrest in connection with the students' disappearance.
According to the police interrogators, the gang members confessed to killing the students because they had mistaken them for members of a rival gang called Los Rojos.
Was there a cover-up?
The relatives of the missing say they are deeply suspicious of the official government version of events, which was presented by the then-Attorney General Jesus Murillo Karam as "the historic truth".
Their doubts have been fuelled further by two independent investigations, which both concluded that there was no evidence that the bodies of the 43 missing students had been burned at the rubbish dump in Cocula, as the official report stated.
However, the independent investigators have not put forward any alternative theories as to what might have happened to the missing students.
What happens next?
The government re-opened its investigation into the students' disappearance in October after the first independent report cast doubts on the official version of events.
That investigation is still under way and as part of it government experts were going to conduct another survey of the rubbish dump in Cocula.
But following the report by the Argentine forensic experts published on 9 February, relatives of the missing demanded the government open other lines of investigation rather than "waste time searching the rubbish dump".