Mexican gangsters who kidnapped a five-year-old boy killed him by injecting sulphuric acid into his heart after his mother publicised his abduction, police said last week.

The brutal murder of Javier Morena, snatched from his family’s fruit stand in a working-class district of Mexico City, marks a horrific escalation in the terror tactics employed by kidnappers. They have increasingly switched their attention to the poor as the wealthy protect their families with squads of armed guards.

Miguel Mancera, Mexico City’s attorney-general, displayed the syringe said to have been used by five gangsters under arrest over Javier’s death. They allegedly confessed that they had wanted to be known as the “Vitriol Gang”, after an alternative name for sulphuric acid, to distinguish themselves from dozens of others. The acid was siphoned off from old car batteries.

The boy vanished two weeks ago while playing at the central market in Iztapalapa, a “barrio” or slum of more than 1m people. His mother, Laura Vega, who lives in a breeze-block house with a corrugated iron roof, feared he had been kidnapped but realised she would be unable to afford a ransom.

After a frantic three-day search through the alleys and child brothels of Iztapalapa, she broke the barrio code of silence and reported Javier’s disappearance to the police.

His picture was broadcast on television, prompting a taxi driver to say he had driven the tearful boy and a teenager claiming to be his brother to a house outside the city. Police raided the building but it was too late.

Mancera said that the moment Javier’s picture had been shown, the kidnappers plunged the needle into the boy’s heart killing him instantly. They buried him on a hill outside the city. The police recovered his body hours later.

Police said the gang had been preparing to demand a £12,000 ransom, but did not know where to send their demand as the boy was too terrified to speak.

Last Monday Javier was reburied in a white coffin near his home. His mother told reporters she did not know why the gang had targeted her family, as they had little money. “He didn’t have to die like that, so far from his family,” she said.

She added that the kidnappers should face the death penalty so that they would suffer “the way my son suffered”. However, capital punishment was abolished in Mexico three years ago.

In 2003, when the British director Tony Scott came to Mexico City to film Man on Fire, in which Dakota Fanning plays a kidnapped child, the abduction rate was 20 victims a month, largely children.

Police now put the rate at 65 a month. The Citizens’ Institute for Crime Studies says that this is an underestimate because few families call in the police. It puts the true figure at closer to 500 a month.

Experts say the crime wave reflects increased violence in the drug trade, in which 4,000 have died in battles between cartels and the police this year.

Although the rich may be protected, they are not immune. Javier Morena’s death recalled the recent kidnapping of Fernando Marti, the 14-year-old son of a sports equipment tycoon, who was snatched at a police checkpoint while being driven to school. It prompted a 100,000-strong protest march at the government’s inability to protect children.

The family paid a £1m ransom but the boy’s decomposing body was found days later in the boot of a car. It now appears that the police uniforms worn by the gang were genuine: the plot is alleged to have been organised by a Mexico City police commander.

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