Mexico has ordered the arrest of the mayor of the city of Iguala, his wife and an aide, charging that they masterminded last month's attack that left six students dead and 43 missing.
Mexican authorities have searched in vain for any trace of the college students who disappeared on September 26, in a case that's sparked national and international outrage, including mass demonstrations that saw the Iguala city hall torched on Wednesday.
"Arrest warrants have been issued for Iguala mayor (Jose Luis Abarca)," as well as his wife and public safety chief, "as the individuals who likely organised the events that took place in Iguala", Attorney-General Jesus Murillo Karam told reporters.
Abarca "gave police the order to confront" students, who were known for frequent protests, so that they would not derail a public event by his wife, the head of a local state children's protection charity.
Authorities say corrupt officials and police worked hand-in-hand with the Guerreros Unidos drug cartel in the attack, which could prove to be one of the worst slaughters that Mexico has witnessed since the drug war intensified in 2006.
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The mayor's wife, Maria de los Angeles Pineda, is a sister of at least three known drug traffickers, and the couple has ties to Guerreros Unidos, authorities say.
Searchers are still desperately combing the area for the missing students by land and air, almost a month later.
Authorities say Iguala's police force shot at buses carrying the students and handed them over to officers in the neighbouring town of Cocula, who then delivered them to the Guerreros Unidos drug gang.
On Wednesday, protesters in Iguala set fire to the city hall building in the latest display of rage over the unsolved disappearances.
It was the second incident in which demonstrators set fire to local buildings in Iguala in as many days.
On Tuesday, 500 teachers set fire to a political party office in the capital of Guerrero state, Chilpancingo.
Authorities have found several mass graves in Iguala but say 28 sets of remains examined so far do not correspond to the students.
This week, the government announced a $US110,000 ($A119,000) reward for information in the disappearance of the students.
A total of 36 municipal officers in Iguala have been arrested in the case, along with 17 Guerreros Unidos members and their boss.
