Mexican federal forces disarmed a southern city's entire police corps and took over security Monday after officers were accused of colluding with a gang in violence that left 43 students missing.
The deployment in Iguala, 200 kilometers (125 miles) south of Mexico City, came after President Enrique Pena Nieto pledged justice in a case that has challenged his vow to tame crime in his country.
Authorities discovered a mass grave on a hill outside Iguala over the weekend containing 28 unidentified bodies, raising fears over the fate of the students, who were last seen in the city more than a week ago.
Authorities say it will take at least two weeks to get the results of DNA tests to identify the corpses. Some bodies were in pieces after being set on fire in a bed of branches.
Witnesses say several students, who are from a teacher training college known as a hotbed of radical protests, were whisked away in police vehicles on the night of September 26 after officers shot at buses the youngsters had commandeered to return home.
Prosecutors say the Guerreros Unidos drug gang participated in the night of violence that left six people dead, 25 wounded and 43 missing.
Two hitmen have confessed to killing 17 of the 43 students, saying they were told by a police commander to head to the scene of the shootings, while a gang leader told them to execute the youths, authorities said.
The government said its new federal police unit, the paramilitary-like gendarmerie, took charge of security Monday.
Following the announcement, army convoys rumbled down the road to the Guerrero state city, where they will man checkpoints.
- Mexicans want justice -
Monte Alejandro Rubido Garcia, the national security commissioner, said the Iguala police officers will be sent to a military base to undergo evaluations while investigators check whether their guns were used in any crimes.
He did not say how many were disarmed but state prosecutors said last week that they had detained 22 officers after taking in 142 for questioning.
Some 30 people in total have been detained, while the mayor and public security chief are on the run and sought by the authorities.
A banner signed "Guerreros Unidos" appeared in the town on Monday demanding the release of the 22 officers within 24 hours, officials said.
While the motive remains a mystery, the case has shed light on Mexico's enduring challenge of cleaning up corrupt and abusive police forces.
"This incident is without a doubt shocking, painful and unacceptable," Pena Nieto said in a televised address at the National Palace in Mexico City.
"Mexican society and the families of the youths who are regrettably missing rightly demand that the incidents be cleared up and that justice be served," he said.
Attorney General Jesus Murillo Karam said more federal investigators would arrive in the city of 140,000 and that he would head there himself soon.
The case could become one of the worst slaughters Mexico has seen since the drug war intensified in 2006, leaving 100,000 people dead or missing to date, and the most horrific since Pena Nieto took office in December 2012.
- Parents skeptical -
But parents of the missing students refuse to believe they may have died, and accused state authorities of lying.
Some of the parents said they were shown pictures of the bodies but that they did not believe that they looked like their children.
Pena Nieto "must deliver them to us alive," Manuel Martinez, a spokesman for the families, said at a news conference at the Ayotzinapa teacher school near the state capital, Chilpancingo.
At the request of student leaders, an independent team of Argentine forensic specialists will help identify the bodies from the mass grave.
The rural college is covered with murals of communist icons like Vladimir Lenin and Che Guevara.
Students from the school have held protests in the past against government policies, but they deny demonstrating in Iguala, saying they went there to raise funds.
But they are known to hijack buses, a common practice for them to get around the state.
Oscar Garcia, 17, said his family was aching over the disappearance of his brother, who wanted to teach Spanish and indigenous languages.
"He wanted to be a bilingual teacher, Mixtec and Spanish," Garcia said.
"As a family, we feel terrible not knowing anything about where they are."