Gunmen using local residents as human shields opened fire on Mexican army patrols investigating fuel pipeline thefts in clashes that left four soldiers and six attackers dead, the military said Thursday.

The confrontations late Wednesday in the central state of Puebla marked an escalation of recent conflicts in which fuel thieves have largely taken control of some towns in the so-called “Red Triangle” area east of Mexico City.

The Defence Department said attackers hiding behind a group of women and children killed two soldiers and wounded a third in the initial confrontation.

“In light of this situation, the soldiers decided not to return fire because the attackers were using women and children as a human shield,” the department said in a statement.

The army called for reinforcements and about 1,000 troops and police were sent in.

Puebla state officials said later that another three attackers had died.

Residents set up road blocks

Angry residents of Palmarito set up road blocks on Thursday to protest the army crackdown, demanding the release of some of the dozen residents detained in the clashes.

Federal Police said hours later that traffic had been restored in both directions along the toll highway between the cities of Puebla and Cordoba.

The army has increasingly faced civilian resistance to drug-eradication patrols, with women and children trying to block soldiers from cutting down opium poppy fields in recent months in the southern state of Oaxaca.

But it is in the pipeline thefts — thousands of illegal taps drilled into state-owned pipelines every year — where local populations have been recruited en masse by gangs that often distribute drugs, steal gasoline and diesel and carry out extortion and kidnapping. They are known in Mexico as “huachicoleros,” a term that refers to illegal or sub—par fuel sold from plastic tanks on roadsides.

While the government’s Pemex oil company no longer releases official figures, 5,574 illegal pipeline taps were found in 2015. By some recent estimates, they cost the company about $1.5 billion per year in lost production.

Some townspeople in Puebla and other states have largely based their local economy on fuel stolen from the pipelines, sometimes collecting gasoline and diesel in buckets when a tap leaks and gets out of control.

‘Poisoning entire families, children’

“This criminal presence is poisoning entire families, children,” Puebla Gov. Antonio Gali said.

A few people have dressed images of the Baby Jesus as the “Nino Huachicolero,” compete with a small plastic tube and a plastic jug to hold fuel.

And mariachi musician Tamara Alcantara has composed the “cumbia de huachicol,” a song in which she defends the residents of her state, which has become known for fuel thefts.

“The huacihcolero is like the devil, everybody knows he is there but nobody has seen him,” goes the song, which she notes is a bit satiric. “I’m speaking about the history of the people of Puebla,” Ms. Alcantara said, noting that huachicolero is just the latest of a long series of derogatory nicknames for residents of the state.

“There has been a lot of attention on Puebla, and the Red Triangle, but all of Mexico, absolutely the whole country, is in crisis,” Ms. Alcantara said.

And perhaps she’s right; according to official figures, Puebla accounted for only about 15 percent of the illegal pipeline taps discovered last year in Mexico. Other states, such as Guanajuato, have more.

Extent of control

But the extent of the gangs’ control in some parts of Puebla is unusual. In March, three Puebla state detectives were kidnapped and killed by a fuel-theft gang known as the “The Bucanans.” Most of the local police and the mayor of the Puebla town of Atzizintla were arrested for allegedly spotting the agents and helping turn them over to the gang.

A raid days later by 500 police and soldiers turned up 87 suspects and 16 rifles, hand grenades and 4,600 rounds of ammunition.

Since then, 647 raids have been carried out across Puebla, leading to the seizure of nearly 1,000 vehicles, almost a half-a-million gallons (1.76 million liters) of stolen fuel, the discovery of 125 illegal taps and the detention of 318 people.

The fuel is often sold from large plastic containers in metal cages in old pick-up trucks parked in vacant lots or along highways. But Mexican security analyst Alejandro Hope said the volume of fuel theft was so large that some of it was likely sold through licensed gas stations.

“They have to attack their market,” Mr. Hope noted. “Part of this stolen fuel is being sold on the legal market.”

“This is no doubt a social aspect, a cultural aspect to this business,” Mr. Hope said, noting that employees at the state-owned Pemex oil company may be in collusion with the thieves.

Rape-murder-assault case

But the fuel thieves already may have roused a stronger crackdown. They are suspected of carrying out one of the more shocking crimes in recent months- the rape-murder-assault against a family travelling on a highway in Puebla Tuesday.

Eight assailants raped a woman and her 14-year-old daughter, killed her nearly 3-year-old son, beat her husband, stole the family’s pick-up truck and left them on the side of a dark highway.

The family had to walk about a mile (1 kilometre) to the nearest toll booth to seek help with the dead boy in their arms.