Two sophisticated tunnels probably built to smuggle drugs into the United States have been found on the border with Mexico.
The tunnels were more than 200m (656ft) long and had lighting and ventilation.
A US agent said their design “wasn’t something that an ordinary miner could have put together”.
More than 150 tunnels have been found crossing the US-Mexico border since 1990; most were crude passageways.
The first tunnel, discovered by the Mexican army on Wednesday, was still unfinished. It started underneath a bathroom sink in a warehouse in Tijuana.
The second tunnel led from an ice plant in the Mexican town of San Luis Rio Colorado to a storage room in what officials described as a “nondescript building” in San Luis, Arizona.
It ran beneath the border at a depth of 16m (55ft) and was lined with plywood.
Counter-narcotics agents believe it had not been in operation for long because there was little wear on its floor.
Special Agent Douglas Coleman, of the US Drug Enforcement Administration, estimated that the tunnel cost $1.5m (£972,000) to build. Three people were arrested in connection with the Arizona tunnel.
The use of underground tunnels for smuggling has increased in recent years as the US authorities have clamped down on overland smuggling activity.
Gunmen opened the doors of the Sol y Sombra discotheque in Uruapan, in the western Mexican state of Michoacan, and threw five human heads onto the dance floor.
As frightened partygoers looked on, the gang left a scrawled message at the scene, announcing the arrival of a new, breakaway drug cartel called La Familia Michoacana, and walked out as coolly as they had entered.
For many, it represented a shocking new degree of brutality by the country’s drug traffickers. It made headlines around the world.
“It generated great fear and terror”, remembers Mr Castellanos, “and then investors started to leave for more secure areas.”
“In the 1990s, the cartels didn’t cut the heads off their victims”, says Samuel Gonzalez Ruiz, a former advisor to the UN Office on Drugs and Crime.
“They used different codes of murder which were more or less established between the criminals,” Mr Gonzalez Ruiz says.
He tells of a well-known hitman who sent out messages by the different ways he shot his victims.
A bullet to the back of the head, for example, meant the victim was a traitor, a bullet to the temple signified he was a member of a rival gang.
Now, however, beheading is a tactic often employed by Mexican drug organisations, in particular by the vast criminal network Los Zetas and their two main rivals, the Gulf Cartel and the Sinaloa Cartel.
Such a violent form of execution is generally associated with the sort of radical Islamist groups who killed US journalist Daniel Pearl in Pakistan, or British civil engineer Kenneth Bigley in Iraq.
Cult of death
But the Mexican context is very different, says Mr Gonzalez Ruiz. He argues the practice comes from Guatemala:
“In 2000, the Zetas began to extend their reach into Central America, and they incorporated into their ranks members of the elite jungle squad, the Kaibiles.”
“The Kaibiles had been trained in using decapitation to threaten the local population since the times of the country’s dirty war (1960-1996).”
>Others see links to a religious cult popular with the drug gangs called La Santa Muerte, or Holy Death.
Some commentators have even drawn comparisons to pre-Columbian human sacrifices by the Aztec and the Mayan civilizations.
Wherever it stems from, the gruesome practice is now a staple in the lexicon of violence of the drug cartels in Mexico.
In early May, 14 decapitated bodies were found in Nuevo Laredo, just over the border from Texas.
During the previous last week, 18 bodies and severed heads were left in two mini-vans near Lake Chapala, an area popular with tourists in western Mexico.
Finally, in one of the most shocking incidents of its kind since the current drug war began, 49 headless and mutilated bodies were left in plastic bags on a road outside the industrial city of Monterrey. (We brought you this story.)
So, beyond the obvious, what are the cartels trying to achieve by butchering their victims in this way?
For the government, it was intended as an uncompromising message of fear and intimidation.
These “reprehensible acts” were designed to “sow fear among the civilian population and the authorities”, said Interior Minister Alejandro Poire the day after the Monterrey atrocity.
“The message is clear: we have no mercy, and we will do whatever it takes to control our territory,” he says.
“You can only call this strategy (of beheading the victims) a terrorist’s strategy. It’s terrorism because it sends a threat to the population: ‘if you don’t allow us to control our illegal business, we will do the same to you’.”
Thinly veiled messages of intimidation aside, the recent atrocities are a grisly reminder of the extent of the cartels’ power in Mexico, and the lengths to which they are prepared to go to hold onto it.