Mexican prosecutors were investigating whether local police officers were involved in the disappearance of 57 students who vanished after deadly incidents in the southern state of Guerrero.
Inaky Blanco Cabrera, the state's chief prosecutor, said the military and state police have been searching for the students -- on the ground and by helicopter -- since the mass disappearance was reported on Sunday.
The students disappeared following a bout of violence in the town of Iguala over the weekend that left six people dead and 25 wounded in different parts of the municipality.
Fellow students from the rural teacher training school of Ayotzinapa broke windows and doors of the state legislature in Guerrero's capital, Chilpancingo, in a protest Monday demanding justice for the dead and the whereabouts of those missing.
Four people died in attacks by groups of unidentified gunmen wearing masks who shot at cars and a bus carrying a third-division football team, killing two players.
Authorities say another two died after municipal police fired at buses that were seized by students protesting discriminatory hiring practices.
State authorities detained 22 municipal officers, including 16 who are believed to have fired their weapons. In all, investigators questioned 162 officers.
"It's undeniable that excessive use of force took place," Blanco Cabrera told reporters in Acapulco, adding that there was "no justification for the use of firearms."
The officers denied attacking the students, while three said they fired in the air, he said.
He added that authorities are investigating whether "forced disappearances" took place, a term used for the abduction of people by security forces.
Traffic cameras caught civilians in the back of a municipal police car while a witness said students were seen in the town's jail cells, he said.
- Mayor was at party -
Authorities are also investigating whether an organized crime group was involved in the other shootings because some unidentified gunmen wore dark clothes and masks and drove black sport-utility vehicles.
The town of 140,000 people appeared calm on Monday, as shops and schools operated normally while military vehicles patrolled the streets.
Soldiers and state police manned checkpoints on the three roads leading to Iguala.
"We heard about the shootings but people are calm, without fear," said newsstand owner Rufina Mendez.
Mayor Jose Luis Abarca came under scrutiny after telling a radio station on Monday that he had not been aware of the shootings the night of the attacks because he was at a ball with his wife.
Blanco Cabrera said prosecutors are "not ruling out" any possible involvement by the mayor or the town security chief in the police violence.
Guerrero is one of Mexico's poorest and most violent states, marked by drug gang turf wars and frequent protests by teachers resisting reforms.
The students' disappearance highlights Mexico's plight of the missing, with more than 22,000 people unaccounted for across the country.
Gangs and security forces have both been accused of making people disappeared, though officials say some of them may be people who voluntarily left home like migrants.