Mexican authorities scrambled Wednesday to find 43 students missing since gang-affiliated police attacked them 19 days ago, as angry protesters demanding their safe return threw rocks at the attorney general's office.
The announcement that the young men were not among 28 bodies found in a mass grave in the southern city of Iguala, where they disappeared on September 26, brought only temporary relief to families and fellow students.
Investigators have been digging two other locations outside Iguala with burial pits holding an undisclosed number of bodies, and officials warn that identifying the victims will take time.
Hundreds of students protested in front of the attorney general's office in Mexico City, hurling stones at its windows and igniting a bonfire in front of the gate.
"They took them alive, we want them back alive," the protesters chanted.
President Enrique Pena Nieto has been under pressure at home and abroad to solve the confounding case in the violence-plagued southern state of Guerrero.
Authorities say Iguala's police force shot at buses carrying the students on September 26 and handed them over to officers in the neighboring town of Cocula, who then delivered them to the Guerreros Unidos drug gang.
Six people died, 25 were wounded and 43 students went missing that night.
"These regrettable events are a test for Mexican institutions and society," Pena Nieto said.
Guerrero's public security department broadened its search, saying it would distribute fliers with pictures of the 43 young men in five states and Mexico City.
Horse-mounted police and rescue dogs were deployed to comb rural, hard-to-reach areas of Iguala, 200 kilometers (125 miles) south of Mexico City.
Some 300 federal police officers and civilian self-defense militias have already been searching for the students for days.
- Relatives hopeful -
Attorney General Jesus Murillo Karam said Tuesday that 14 Cocula police officers were arrested in the case, joining 26 detained colleagues from Iguala.
The motive remains under investigation.
The students, who are from a Guerrero teacher training college known as a bastion of radical protests, went to Iguala to raise funds and seized buses to go home, a common practice among the aspiring educators.
Relatives of the students refuse to believe they are dead and have led protests demanding their safe return.
"It's possibly a kidnapping, but as time goes on, there is less hope that they will be found alive," Javier Oliva, a security expert at the National Autonomous University of Mexico, told AFP.
Oliva said it would be complicated to keep 43 people incomunicado for so long, without them being seen by anybody.
"Just think about how they are fed. Even if it's one daily ration, where do you get food for 43 people," he said.
- 'Horrified and ashamed' -
In addition to a missing persons case, the authorities are now discovering previously unknown victims as they dig up the mass graves in Iguala.
At least 80 corpses have been discovered around the city of 140,000 people this year alone, revealing the extent of the horrors committed by criminals with impunity.
"They could be victims of fights (between gangs) or extortion, kidnappings and organ trafficking," Oliva said.
Authorities say the Guerreros Unidos gang, which is an offshoot of the bigger Beltran Leyva drug cartel, was engaged in turf wars with rivals while infiltrating local authorities.
The wife of Iguala's mayor's is the sister of two slain members of the Beltran Leyva cartel. The couple and the city's police chief are on the lam.
Mexicans have endured a drug war that has left 80,000 people dead and more than 22,000 others missing since 2006, but their exposure to atrocities has not made them insensitive to the events in Iguala.
Mexicans are "horrified and ashamed," Roy Campos, president of pollsters Consulta Mitofsky, told AFP.
The case has further tarnished the political class "because it has shown a cohabitation between mafias and politicians," Campos said.