A dozen state police officers have been arrested for allegedly killing 19 people whose bodies were found shot and burned near the US border late in January.
Tamaulipas state Attorney General Irving Barrios Mojica said 12 officers were in custody and face charges of murder, abuse of authority and making false statements.
The killings revived memories of the gruesome 2010 massacre of 72 migrants near the town of San Fernando in the same gang-ridden state.
The attorney general did not say what motive the officers might have had, though corrupt local and state police in Mexico are often in the pay of drug cartels.
Cartels in Mexico often charge migrant smugglers for crossing their territory, and kidnap or kill migrants whose smugglers have not paid, or paid a rival gang.
The bodies were found piled in a charred pick-up truck in Camargo, across the Rio Grande from Texas, in an area that has been bloodied for years by turf battles between the remnants of the Gulf cartel and the old Zetas cartel.
Authorities have said four of the dead have been identified so far - two Guatemalans and two Mexicans.
Their names have not been released by officials, but relatives of one of the dead Mexicans said he worked as an immigrant trafficker.
Of the 19 bodies examined, 16 were male, one was confirmed as female and the two others were so badly burned their gender has not yet been determined.
The forensic results confirmed the fears of families in a rural indigenous farming community in Guatemala who said they lost contact with 13 migrants as they travelled toward the United States.
Relatives of migrants from Guatemala's province of San Marcos are so convinced 13 of the corpses were their loved ones that some of the families have already erected traditional altars to the dead, with flowers and photographs.
The truck holding the bodies had 113 bullet impacts, but almost no spent shell casings were found at the scene.
Initially, that led investigators to speculate the shootings may have taken place elsewhere, and the truck driven to the spot where it was set on fire.
But Barrios Mojica said the police officers charged in the killings knew their shell casing might give them away.
"There is growing force behind the hypothesis that the crime scene was altered, due to the absence of casings," he said.
The massacre is the latest chapter in Tamaulipas' history of police corruption.
Most towns and cities in the state saw their municipal police forces dissolved years ago, because officers were often in the pay of the cartels.
A more professional state police force was supposed to be the answer, a belief that came crashing down with the arrests announced on Tuesday.