Mexico missing students: 'No impunity' vows Pena Nieto
Mexican President Enrique Pena Nieto has vowed to identify and punish those responsible for the recent disappearance of 43 students after clashes with police.
In a televised address, he described the incident in the city of Iguala as "shocking, painful and unacceptable".
Shortly after the speech, federal forces disarmed municipal police and took over security in the city.
The students went missing after a protest over job discrimination.
Police opened fire on their buses as they arrived in the city, in the state of Guerrero, on 26 September.
Twenty-two police officers are being held in connection with the shooting.
"Like all the Mexican society, I am shocked by this situation and I can assure you that there will be no impunity," Mr Pena Nieto wrote later in his Twitter account.
Concerns about the wellbeing of the missing students increased on Saturday, when clandestine graves were discovered on the outskirts of Iguala following an anonymous tip-off.
Twenty-eight burnt bodies have been retrieved from the pits.
Forensic experts are now trying to establish whether the bodies are those of the missing students. Results are expected within two weeks.
The group of students had travelled to the area to take part in a protest over discrimination against rural teachers.
Iguala lies about 200km (120 miles) south of the capital Mexico City.
Three people were killed when police shot at the buses. Another three were shot dead in an incident that took place a few hours later, on 27 September.
Witnesses said that most of the survivors - all trainee teachers - were bundled into police vans before disappearing.
Some went into hiding and contacted their relatives after several days, still fearing for their lives.
State prosecutors said local officials and police officers could have links with criminal gangs operating in Guerrero state.
Arrest warrants have been issued for the mayor of Iguala, Jose Luis Albarca, and his security chief who are both considered fugitives from justice.
Mexico correspondent Will Grant says the incident has highlighted the extent of the collusion between criminal groups and officials in much of the country.