Twelve police officers have been arrested in Mexico following the discovery of 19 charred corpses near the US border, Tamaulipas Attorney General, Irving Barrios, said on Tuesday.
The Tamaulipas state police are accused of murder, abuse of power, and making false statements.
The bodies were discovered on 22 January in a burned-out pickup truck next to other burned cars in the town of Santa Anita in north-eastern Mexico.
The people had been shot dead, according to media reports. Among them, according to Barrios, were migrants from Guatemala.
The media had reported that almost all of the victims were Guatemalans.
Tens of thousands of Central Americans every month flee poverty and violence in their homeland to seek a better life in the US.
One of the four dead identified so far was a Mexican migrant smuggler, according to Barrios.
The possible involvement of other police officers, as well as criminal groups smuggling Central American migrants into the US and fighting for control of the region, is still under investigation, Barrios said.
The scale of daily violence in Mexico has been immense since the so-called drug war broke out in late 2006.
Currently, nearly 100 murders a day are recorded in the country of nearly 130 million people.
In addition, more than 82,000 people are believed to have disappeared.
Much of the bloodshed is the work of cartels and gangs involved in drug smuggling to the neighbouring US, but they also make money through kidnapping and extortion and often have links to security forces.
Only about 3 per cent of crimes reported in Mexico are solved, according to figures from the think tank IEP.