Police in the southern Mexican state of Guerrero are searching for 58 students who have been missing since Friday.
The students from a teacher training college disappeared after deadly clashes erupted between them and security forces during a protest.
Six people were killed and 17 injured when police and unidentified gunmen shot at the protesters and opened fire on a bus in the town of Iguala.
Some 22 officers are being held in connection with the shooting.
The students from the Ayotzinapa teacher training college were protesting against what they say are discriminatory hiring practices for teachers which favour urban students over rural ones.
Members of the student union said they had been protesting and tried to hitchhike rides back to their college on local buses.
But the municipal police said the students were trying to seize the buses by force and that is why they gave chase.
Shots were then fired at a bus carrying third division football team Los Avispones, which the gunmen presumably mistook for a bus seized by the students.
It is not clear whether those shooting at the bus were part of the police force or not.
The bus crashed, killing the driver and one of the players.
Four other people, two of them students, were also killed by gunfire.
Student activists have accused the security forces of holding the missing students illegally. But the Guerrero state authorities said no students were being held.
Police said the missing may have fled into the surrounding hills when the shooting started.
A helicopter has been deployed to search for them.