Ayotzinapa (Mexico) (AFP) - Mexico's president vowed Monday to punish those responsible for the disappearance of 43 students who vanished in a night of violence blamed on crooked police tied to a drug gang.

President Enrique Pena Nieto made his pledge after the weekend discovery of six adjoining pits containing 28 unidentified bodies outside the Guerrero state city of Iguala, where the students were last seen 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 found some 200 kilometers (125 miles) south of Mexico City. Some were in pieces after being set on fire in a bed of branches.

Two gang hitmen linked to Iguala's municipal police force have confessed to killing 17 of the 43 students in the same Pueblo Viejo district where the clandestine grave was found, authorities say.

But families of the missing refuse to believe that the students are dead, saying "they took them alive, we want them back alive."

While the motive remains a mystery, the case has shed light on Mexico's enduring challenge of cleaning up corrupt and abusive police forces.

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.





- Mexicans demand justice -





Pena Nieto, whose pledge to reduce violence has been challenged by relentless gang activity in several Mexican states, said he instructed his security cabinet to help in the investigation.

"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.

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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.

Some 30 people have been detained over the shootings, including at least 22 police officers, while the mayor and public security chief are sought by the authorities.

The federal government deployed members of its new paramilitary gendarmerie police force to Iguala on Monday to take over security.

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.

A banner signed "Guerreros Unidos" appeared in the town on Monday demanding the release of the 22 officers within 24 hours, officials said.

Two of the gang's hitmen told investigators they were ordered by Iguala's public security director to head to the site of the bus shootings, prosecutors said.

The hitmen said they then received instructions from a gang leader known as "El Chucky" to take the students and kill them.





- Parents skeptical -





But parents of the missing students say state authorities are 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 take over the issue. He must deliver them to us alive," Manuel Martinez, a spokesman for the families, said at a news conference at the Ayotzinapa teacher school.

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 near the state capital Chilpancingo.

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."