None of the 43 students who had gone missing in Mexico were among the 28 bodies found in the first set of mass graves in the southern state of Guerrero, Mexico's attorney general said.

Jesus Murillo Karam said on Tuesday that there was still no sign of the college students who disappeared September 26, after violent incidents blamed on police in the city of Iguala.

"We have some (DNA) results for the first pits and I can tell you that they do not correspond to the DNA that relatives of these young men have given us," Murillo told reporters.

At the same time, Karam announced the discovery of another mass grave and that 14 more police officers were

arrested and have confessed to participating in the disappearance. The 14 are from the police force of Cocula, a town neighbouring Iguala.

Authorities have said police involved in "disappearing" the students were working in conjunction with a local drug gang.

Al Jazeera's Rachel Levine, reporting from Guerrero state capital Chilpancingo, said that families of the missing students have not been told of the result of the DNA test.

"Families are very frustrated that they are being kept in the dark," she said, adding that the families continue to demonstrate outside the Guerrero state government complex.

One forensic expert who works with federal investigators said charred remains like those recovered at the first mass grave sites found outside Iguala can leave very little DNA for testing.

"If a bone is burned at more than 300 degrees, it's almost impossible to identify because the collagen is burned. Because of that, criminal organisations started to adopt that technique." said Jorge Arturo Talavera, head of the bio-archaeology team at the Mexico's National Institute of Anthropology and History.

He said the way the remains are exhumed also can affect identification.

If they are pulled out in a hurried manner, other identifying articles like jewelry or teeth get discarded.

Experts still are testing remains recovered from other mass burial sites found near Iguala, but they have not given information on what was found there.

Police raid

Meanwhile, an alleged leader of that drug gang, Guerreros Unidos, killed himself during a confrontation with Mexican security forces on Tuesday, a day after protesters demanding an investigation into the students' whereabouts burned government buildings in Chilpancingo, the capital of Guerrero.

Federal forces, which include federal police and military personnel, were carrying out an operation to capture Benjamin Mondragon, or "Benjamon," the alleged head of the Guerreros Unidos gang in the neighbouring state of Morelos, when the suspect killed himself.

The official said it was unclear whether Mondragon was involved in the students' disappearances.

The disappearance of the students in Guerrero has triggered massive protests across the country, calling on President Enrique Pena Nieto to find the students, and go after the suspects.

At least one detained gang member has said another Guerreros Unidos leader known as "El Chucky" had ordered him to kill 17 students.

In a speech Tuesday, President Enrique Pena Nieto said the disappearances "are without doubt a topic that will lead the Mexican government at all levels to take actions that will prevent events like those in Iguala from ever occurring again".

Police are also looking for the mayor of Iguala, Jose Luis Abarca, in order to question him. His wife's brothers include two deceased top-level members of the gang, which split off from the Beltran Leyva cartel.

The gang controlled drug routes in Guerrero and Morelos, the neighbouring state to the north.