DNA tests have confirmed that none of the 28 bodies found in a mass grave in restive southwestern Mexico are those belonging to 43 students who went missing after protestors clashed with local police in the violent state of Guerrero three weeks ago in incidents which also resulted in six deaths, Mexico's attorney general said on Tuesday. The students were from a special kind of rural university in the town of Ayotzinapa, near Iguala.

Four other recently discovered grave sites are now being investigated for further tests, attorney Jesus Murillo Karam said at a press conference on Tuesday, adding that none of the charred remains found in the first mass grave matched genetically with the missing students, Reuters reported.

"What I can say is that the first mass grave we found, the very first ones from where we already have results, I can say the (remains) don't match the DNA of the families of these young people," he said.

"On Sept. 26-27, Iguala city police attacked a group of students rallying to protest against government policies. Six people were killed, more than two dozen injured and more than 50 students vanished. About 15 eventually were found hiding in their homes, but 43 remained missing," the Los Angeles Times reported.

The Aytozinapa Normal school, attended by the missing students, is known for militant and radical protests that often involve hijacking buses and delivery trucks, according to Fox News.

Within days, 22 police officers were arrested for what prosecutors said was the unjustifiable use of excessive force. They are believed to have been penetrated by criminal organizations and a drug gang, known as the Guerreros Unidos, at whose behest the police might have been acting.

"On Tuesday, police said the gang's leader, Benjamin Mondragon, had killed himself when he was about to be arrested during an operation by Mexican security forces in the state of Morelos," BBC News reported.

Since the burial sites were found, an additional 14 people have been arrested, said Vidulfo Rosales, a lawyer for a local human rights group who is assisting the families of the missing students, adding that he could not identify them.

Meanwhile, more than 20,000 people are registered as having disappeared in Mexico in the past eight years. Most of them have never been found.