The disappearance of 43 students from a teacher training college in the Mexican town of Ayotzinapa on 26 September has outraged many Mexicans.
Pictures of the 43 can be seen throughout Mexico at demonstrations in support of the missing students.
The trainee teachers went missing after holding a protest in the nearby town of Iguala, about 200km (125 miles) south-west of the capital, Mexico City.
Municipal police say they intercepted the students on the orders of the local mayor, Jose Luis Abarca, who wanted to prevent them from interrupting a speech his wife was giving that day in Iguala.
Three students and three people in nearby vehicles were shot dead when the police opened fire on the students on a road outside Iguala, in Guerrero state.
Their killing triggered small protests in Iguala, but it was the disappearance of their fellow students which captured the headlines.
Officials say they were seized by municipal police officers as they were trying to flee the shootout. After briefly detaining them, the officers are reported to have handed them over to a local drug gang.
More than 70 people, including police officers, local officials and members of a drug gang called Guerreros Unidos are under arrest for allegedly taking part in the disappearance of the students.
Mexico's Attorney General says members of the drug gang took the students to a landfill site, where they killed them and burned their bodies.
Remains found at the site have been sent to a forensic lab in Austria for analysis. The tests could take weeks, experts said, and warned that it would be difficult to extract DNA from the badly burnt remains.
Relatives of the missing say they refuse to accept the students are dead until independent tests confirm it and have promised to continue their search.
Anger about the links the disappearance has exposed between the drug gang, the police and the local authorities has triggered a series of protests, some of which have turned violent.
The protests have since spread from Iguala to the state capital, Chilpancingo, and beyond.
On Wednesday, protesters set fire to vehicles outside the Guerrero state congress building in the city of Chilpancingo.
And at the Monte Alban archaeological site in the southern state of Oaxaca, protesters unfurled banners reading: "Until we find them."
But there were also protests as far away as the Netherlands, where supporters of the Mexican football team held up posters representing the disappeared during a friendly game between Mexico and the Netherlands in Amsterdam.
One of the most popular slogans in support of the students is: "They took them alive, we want them back alive."
Others called on President Enrique Pena Nieto to resign.