The brutal public killings that began about five years ago have worsened as Mexican drug cartels try to outdo each other in their quest to scare off rivals, authorities and would-be informers.
Killings grow more gruesome as Mexican drug cartels try to out-shock
A woman's decapitated body is left at a border city's monument to Columbus, the head atop a computer keyboard with a sign saying she was killed for blogging about drug traffickers.
The severed heads of five men are dumped outside an elementary school in Acapulco, and two more near a military base in Mexico City days later.
That was just in the past three weeks.
The brutal public killings that began about five years ago have worsened as Mexican drug cartels try to outdo each other in their quest to scare off rivals, authorities and would-be informers - and still stun Mexicans increasingly numbed to the gory spectacles.
Clark McCauley, a psychology professor at Bryn Mawr College in Pennsylvania and an expert on terrorism, said: "These gangs have to keep escalating because they want the shock value, but the shock value wears off. Now, to get a headline you have to get more heads, or more bodies or do something more horrific."
Latin American drug lords have long turned to grisly killings and torture. At the height of its powers in the 1990s, the Juarez cartel used to cut off the fingers of snitches and shove them down their throats, a practice that other cartels soon followed.
The current show of savagery began in April 2006 when two police officers were decapitated; their heads dripping blood were left in Acapulco, where four alleged members of the Zetas drug cartel had been killed in a shoot-out with police. Along with the heads was a sign that warned, "So that you learn to respect". The Zetas are a gang of drug smugglers and hit men led by deserters from a Mexican army unit, who for many years were assassins for the cartel.
Five months later, the La Familia cartel rolled five human heads purportedly belonging to Zetas across a dance floor in the state of Michoacán. An attached note said La Familia "doesn't kill for money, doesn't kill women, doesn't kill innocents, just those who should die," an apparent warning for the particularly violent group.
Since then, drug traffickers have plunged into even more gruesome tactics. They have tied victims to overpasses and shot them to death during rush hour as sickened motorists watched. Some have decapitated people and then posted videos of it on the internet.
"In terms of the cruelty, it is the Zetanisation of the country because the Zetas were the first to introduce these ghastly tactics into Mexico," said George W Grayson, a Mexico expert at the College of William and Mary in Virginia, who has written several books about the rapidly expanding drug cartel.
The Zetas are the game-changers.
Officials blame a group calling itself the "Zeta Killers" for dumping 35 bodies on a busy boulevard in the city of Veracruz on September 20. They said the group also killed 32 people whose bodies were found at three houses in the area on Thursday.
Last week, police in Mexico City found two severed heads on a street near a major military base accompanied by a note referring to the "Mano con Ojos," or "Hand with Eyes," drug gang. Motorists called the police after spotting one of the heads on the hood of an SUV.
"If you want to have cartel cred," said Mr Grayson, "you have to show you can carry off any act at any time and go as far as your enemy."
Public Safety Secretary Genaro Garcia Luna has said Mexican drug traffickers copied the terror tactic from the Al Qaeda in Iraq after it posted videos on the internet of the decapitations of Americans. He said the cartels were using Al Qaeda's methods to pressure the government to halt its crackdown against drug traffickers, which had fractured many of the gangs.
Authorities have also said that in 2005, the Zetas began enlisting "Kaibiles," former members of a Guatemalan counterinsurgency unit, to train newly recruited foot soldiers. The Kaibiles were known for massacres during the Guatemalan civil war that ended in the mid-1990s.
Very few of the killings result in arrests or convictions, so the only deterrent is revenge by another cartel. In the five years since the beheading of the two Acapulco police officers, decapitations have become almost weekly occurrences and a prime terror tactic.
Residents in some cities caught in the turf battles are adapting to living with violence, said Oscar Galicia, a psychology professor at Iberoamerican University in Mexico City who specialises in the study of violent behaviour.
In the city of Monterrey, where the Zetas are fighting the Gulf drug cartel, many people don't go out at night in certain neighbourhoods, they avoid night clubs and bars and have added extra locks to their doors at home.
More worrisome is that the prolonged violence is creating a sense of helplessness among Mexicans, who are becoming increasingly numb to what's happening, Mr Galicia said.
"Now if it is not 20 bodies, it does not get our attention and that is terrible and really dangerous for our society because we are becoming as desensitised as the criminals," he said.