BERNANDINO HERNANDEZ, FILE

FILE - In this Oct. 2, 2011 file photo, relatives weep after gunmen opened fire on a taxi killing the driver and the passenger in the Pacific resort city of Acapulco, Mexico. The brutal public killings that began about five years ago have worsened as Mexican drug cartels try to one-up each other in their quest to scare off rivals, authorities and would-be informers _ and still shock Mexicans increasingly accustomed to the gory spectacles.

Masked gunmen dump the bodies of 35 slaying victims during rush hour as terrified motorists watch and tweet friends to avoid the avenue in a Gulf coast city. A couple of weeks later, 32 more corpses are found nearby in three houses.

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 last three weeks.

The brutal public killings that began about five years ago have worsened as Mexican drug cartels try to one-up each other in their quest to scare off rivals, authorities and would-be informers — and still stun Mexicans increasingly numbed to the gory spectacles.

"These gangs have to keep escalating because they want the shock value but the shock value wears off," said Clark McCauley, a psychology professor at Bryn Mawr College in Pennsylvania and an expert on terrorism. "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 tactics. 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 the resort city of Acapulco, where four alleged members of the Zetas drug cartel had been killed in a shootout 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 an elite Mexican army unit, who for many years were assassins for the Gulf cartel.

Five months later, the La Familia cartel rolled five human heads purportedly belonging to Zetas across a dance floor in the western state of Michoacan. 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 retaliation 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 alive and then posted videos of it on the internet.

"In terms of the cruelty, it's 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 Williamsburg, 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 Gulf coast city of Veracruz on Sept. 20. They say the group also killed 32 people whose bodies were found at three houses in the area on Thursday.

On Monday, 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 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-Qaida in Iraq after it posted videos on the internet of the decapitations of Americans. He said the cartels are using al-Qaida's methods to pressure the government to halt its crackdown against drug traffickers, which has fractured many of the gangs.

Authorities have also said that in 2005, the Zetas began enlisting "Kaibiles," former members of an elite 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.

The practice dates back at least 2,000 years, said Dr. Michelle Bonogofsky, an bioarchaeologist who edited two books on the significance of of the human head in different cultures, from skull collection to decapitations.

"One of the worst things you can do to the body, in some instances, is to desecrate or dismember it and historically, this has been used by kings and various other groups to establish control," Bonogofsky said. "This could be tied to the religious belief that you need your body intact to be resurrected."

Residents in some cities caught in the bloody turf battles are already adapting to living with violence, said Dr. Oscar Galicia, a psychology professor who specializes in violent behavior at Iberoamerican University in Mexico City.

In the northern city of Monterrey, where the Zetas are fighting the Gulf drug cartel, many people don't go out at night in certain neighborhoods, they avoid night clubs and bars and have added extra locks to their doors at home.

"What people are doing in Monterrey is adapting," he said.

More worrisome is that the prolonged violence is creating a sense of helplessness among Mexicans, who are becoming increasingly numb to what's happening, Galicia said.

"Now if it's not 20 bodies, it doesn't get our attention and that's terrible and really dangerous for our society because we're becoming as desensitized as the criminals," he said.