Mexico City (CNN) -- Traversing Mexico's highways as a bus driver has always been a dangerous job, says Enrique, whose route takes him from central Mexico to Tamaulipas, on the border with the United States.
"But never like now, that I can assure you," said Enrique, who has been a driver for three years.
His view coincided with other bus drivers who spoke with CNNMexico, who drive through drug cartel territory, and asked for anonymity because of fear of retribution for speaking on the record.
Assaults, armed men, crossfire, bodies or human heads on the side of the road, form part of the daily stories that drivers have told each other in the past two years. It is a reality that came to light with the discovery last month of mass graves in the town of San Fernando, Tamaulipas, that possibly contained the bodies of kidnapped bus passengers.
In addition to the development of kidnappings and assaults of passengers, bus drivers are extorted so that they may transit freely on the highways.
"In some places these people (drug cartels) ask for money in exchange for our security. ...They ask us for money to supposedly travel safely on the roads," said another bus driver.
How much money do they demand? "From 200 to 1,000 pesos ($17-$87)."
What guarantees are there that with that they will leave you alone? "There is no guarantee, but there is nothing we can do when they arrive with a gun, intimidate you, and tell you they belong to this or that criminal group."
"Suddenly, on the street, they watch us, they arrive in trucks, it's easy to recognize us because of the uniform," the driver continued. "They approach and say, 'I come from this place and belong to this group. We are asking for your cooperation, or else there will be problems -- consequences."
Carlos, a bus driver with 10 years of experience, said that in his travels through Tamaulipas, he has been witness to armed confrontations between police and traffickers, and in more than one occasion has seen bodies strewn or mutilated on the highways.
One time, he had to maneuver his bus to avoid running over two groups who were firing on each other right on the highway.
"I accelerated and dodged the bullets," Carlos said. "The passengers were scared. Some cried, others prayed."
"I almost ran over all of them because they passed by running. They didn't care about the size of the bus, or about who was on the highway, they crossed it in plain morning. We accelerated to avoid that a bullet would hit the bus," he added.
In more than a decade of driving through the Tamaulipas cities of Reynosa, Matamoros, Ciudad Victoria and even San Fernando, Carlos said that he has never seen anything like what he has witnessed in the past months.
"Not even when we had to cross the mountains. Now, not even the highways are safe," he said.
Another driver, referring to the kidnapping and apparent killing of bus passengers in San Fernando, said, "We had been witnesses to the violence in the country. Today we are protagonists. It touched us, they reached us."
Government analysts and the country's National Human Rights Commission, posit that Tamaulipas, like other northern states, two phenomena have converged and resulted in the recent violence: drug trafficking and migration.
The drivers interviewed agreed that the routes in the north part of the country have become desirable for criminals because a majority of the bus passengers are migrants destined for the United States.
"(These are) people who are carrying money or who comes from (the United States) with dollars and gifts for their families," the driver says.
According to the human rights commission, last year more than 11,000 migrants were kidnapped in Mexico during their journeys to the United States.
And, according to the National Security System, in 2010 there were 2,142 robberies on highways in all of Mexico. States like Michoacan and Tamaulipas went from having zero highway assaults last year to 11 and 28, respectively, so far this year.
"There were always assaults on the highways, but they were sporadic and another type of crime. They were people dedicated to robbery, but not drug trafficking," said another bus driver, who identified himself as Rafael.
"We can't explain what is happening. We suppose that it is the reaction of organized crime to the job the government is doing to eliminate them, but at the same time we see a lot of corruption among the police," he said. "
The cities of Matamoros, Reynosa and Laredo, in the northeast of the country, are the considered the most dangerous routes.
Asked if they want to continue traveling north, a bus driver answers that, "for necessity, for work, by orders, but it is unsafe. In a tone of jest they ask us if we are carrying our bulletproof vest and helmet. Once someone tells you that, you start thinking, 'I hope nothing goes wrong.'"