Ayotzinapa (Mexico) (AFP) - Desperate relatives of 43 missing university students have demanded a meeting with Mexico's President Enrique Pena Nieto, after nearly a month of searching that has yielded no trace of their loved ones.

Federal prosecutors have searched in vain for clues to the whereabouts of the teacher trainees, who disappeared on September 26 in the city of Iguala.

The students were said to have been rounded up by police with alleged drug-gang ties accused of killing six of their classmates.

Authorities say local officials and police worked closely with the Guerreros Unidos drug cartel in carrying out the attack and abduction of the students, which could prove to be one of the worst slaughters of Mexico's bloody eight-year drug war.

Frustrated relatives, in a statement late Friday, called on Pena Nieto to meet them face-to-face, urging him to "listen directly to the voice of the parents."

The students' friends and families have expressed distrust in the investigation being led by state prosecutors.

Officials said late Friday that the number of bodies found in mass graves in Mexico has risen from 30 to 38, but said none of the remains appeared to belong to the young adults, who were students at a rural teachers' college.

The latest grim find brings to more than 80 the total sets of remains found in the region since the beginning of the year.

The case has cast a harsh spotlight on the epidemic of violence in Mexico, where tens of thousands of people -- mostly victims of drug gang violence -- have disappeared over the past decade without a trace.

It also has called attention to the murky relationships between corrupt officials, police and Mexico's drug cartels in Mexico.

The European Union, the United Nations' Office of the High Commissioner for Human Rights and other international bodies have urged the Mexican government to redouble its efforts to find the missing students.

Meanwhile, the UN rights office in Geneva on Friday urged Mexico to let the United Nations make direct contact with families of the disappeared so that it can carry out an independent investigation.

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The case led this week to the resignation of the governor of Mexico's Guerrero state, as well as the issuance of an arrest warrant for the mayor of the city of Iguala, his wife and an aide, who are alleged to have masterminded last month's attack.

Protesters armed with sticks, pipes and stones stormed offices Friday belonging to the Guerrero state secretary of education in Chilpancingo, the capital, demanding the students' safe return.

The demonstrators -- in the most recent of many protests demanding the students' safe return -- smashed windows and set fire to documents and furniture, which firefighters extinguished. No one was injured in the episode.

More than 22,000 people have disappeared since the government sent in troops to fight the drug-trafficking cartels in 2006.