Authorities were searching on Monday for 57 students reported missing after weekend violence left at least six people dead and 25 wounded in the southern Mexican state of Guerrero.
On Friday, gunmen had fired on a demonstration the students were attending.
Twenty-two police officers from the city of Iguala were detained in connection with the incidents, Guerrero state prosecutor Inaky Blanco said. He said they denied responsibility but several had been identified by students.
Blanco Cabrera said that witnesses identified the gunmen as local police officers in Iguala, where the protest occurred.
Mr. Blanco Cabrera described the episode as an instance of “undoubtedly excessive force” by the officers.
The federal interior secretary, Miguel Ángel Osorio Chong, called the conduct of the police “incredible” and said federal authorities would join the investigation.
Witnesses said police officers had put the missing students on a bus and escorted them from the scene.
The mayor of Iguala, Jose Luis Abarca Velazquez, and his public safety secretary were being summoned to appear before authorities, Blanco added.
On Monday, masked demonstrators marched through the state capital, Chilpancingo, pelting the glass-window entrance to the state congress building with rocks.
The violence on Friday night and Saturday in Iguala began when police clashed with students who had seized three buses in a protest.
The governmental National Human Rights Commission has said it is investigating.
No suspects were named. “We cannot deny, that there was an excessive use of force,” Blanco said in a press conference that was broadcast live on Mexican news channels. “We are planning to launch criminal indictments against those who are responsible for it.”