The United States is growing concerned that some of its billions of dollars of aid sent to Mexico might be funding security forces accused of killing innocent or unarmed civilians, internal military documents show.
Two recent cases of Mexican authorities firing on civilians have caught the attention of US Northern Command, the Pentagon post that oversees US military operations in North America.
In an email dated October 15, the command's Human Rights Working Group told senior military authorities that Mexican military units might be ineligible for U.S. aid if allegations of abuses are proven true.
In response, US military officials later decided that no aid would be offered to the Mexican battalion accused of carrying out a June 2014 military massacre of civilians known as the Tlatlaya case.
On June 30, Mexican soldiers executed 22 unarmed people in an abandoned shed in the municipality of Tlatlaya, in the State of Mexico, news reports and authorities say. Soldiers also apparently altered the scene of the killings, placing weapons near the bodies of dead civilians.
Blood stains on walls suggested the victims were lined up and shot at close range.
The area where the massacre took place is considered firmly under the grip of warring criminal gangs, and military authorities said the dead were suspected members of an organized crime group.
But mounting media evidence forced authorities to admit that a massacre likely occurred. A survivor of the attack told Esquire magazine the people in the shed surrendered to the soldiers but were killed anyway. Graphic photographs published by VICE News offer striking evidence of the scene being manipulated.
"If implicated in a gross human rights violation, the entire military zone and 10,000 personnel would be ineligible for U.S. security assistance," the Northern Command message said.
"There is greater acceptance that the military was involved in wrongdoing," it added.
Mexico's Secretariat of Defense — or Sedena, the army's formal name abbreviation — divides the country's territory into military zones, where battalions are stationed on a rotatory basis.
Seven Mexican soldiers from 102nd Battalion are facing charges related to the killings in Tlatlaya, which is located within Mexico's 22nd Military Zone.
But by September, the case was eclipsed by the police attacks on buses carrying teachers college students in Guerrero state. Six unarmed people were killed and 43 students from the Ayotzinapa Normal School were disappeared in that attack, which drew international media scrutiny.
Dozens of bodies were later unearthed in unmarked mass graves in the hillsides around Iguala, the city where the Guerrero attacks took place. But none of the missing students were identified among those remains, a fact that sparked an alert for the Pentagon human-rights analysts.
"None of the 28 bodies identified thus far are the remains of the students, raising alarming questions about the widespread nature of cartel violence in the region and the level of government complicity," the October email said.
The 1997 US measure known as the "Leahy Law" prohibits the government from offering funding or training to foreign security agencies committing gross human rights violations.
The Northern Command document was published on Tuesday by the National Security Archive, through a Freedom of Information Act request.
It follows previously revealed US military documents discussing the Tlatlaya massacre that point to growing concern that the United States aid package known as the Merida Initiative is possibly funding human rights abusers.
A third government massacre not mentioned in the documents occurred on January 6 in daylight hours in the city of Apatzingan, Michoacan.
Mexican federal police killed 16 unarmed civilians, including members of a family, and altered one of the shooting scenes, according to an investigation published by VICE News.
No officials are under arrest or being investigated for the Apatzingan deaths.
Mexico has received $2.5 billion in aid from the US for its fight against drug cartels since 2008. In that same period, scores of civilians have been killed, kidnapped, or tortured as cartels fractured and authorities poured more weaponry in the fight against them.
Tens of thousands of people remain missing, but the government has wildly fluctuated the official figure, which now stands at 23,000, although the number is not verified. More than 281,000 people in Mexico are internally displaced by the drug war.
'What prompted the fact that soldiers felt it was perfectly acceptable to open fire on unarmed individuals?'
In the Tlatlaya case, authorities said none of the seven soldiers accused of the massacre had received training by the United States, but five other personnel in the 102nd Battalion had.
US officials in January quietly said they would be "suspending" aid to the 102nd Battalion in Mexico, pending results of the investigation, another released document shows.
It was not clear if this was the first targeted suspension of aid for a Mexican military unit, or if other units have also been designated as ineligible for aid. Sedena's director of communications did not reply to a request for comment.
US military assistance and active intervention in Mexico is a touchy subject for both governments.
The United States publicly supports Mexico's military-led campaign against drug traffickers, and rarely if ever offers criticism or rebuke for a growing list of abuses and injustices said to be occurring by government hands. On the other side, Mexico's political climate carries a nationalist sentiment, so US support is downplayed or ignored in public debate.
"The overall US support for the Mexican security strategy is a lot more political support that has for the most part been 'Gung-ho, good job, Mexico'," said Maureen Meyer, a senior associate on Mexico at the Washington Office on Latin America.
"It's important to show that in certain cases the Leahy Law does work," Meyer told VICE News.
A wave of citizen protests held around the world after the Ayotzinapa massacre has prompted renewed public scrutiny on the nature of the bilateral security relationship. Online campaigns circulating in the United States have called on President Barack Obama to cease funding Mexican security forces in the drug war.
The Tlatlaya case is also considered a test of a recent military code reform that allows soldiers accused of human rights abuses to be prosecuted in civilian courts rather than in military tribunals.
Mexico's government on Wednesday said it would be paying reparations to 13 families of some of the 22 victims in the Tlatlaya massacre.
"This will be a test," Meyer said. "Did the general have any role in this? Was there any chain of command permitting this? What prompted the fact that soldiers felt it was perfectly acceptable to open fire on unarmed individuals?"
"The mindset is that it's OK to do this because these people do not have due process guarantees," she added.
Follow Daniel Hernandez on Twitter @longdrivesouth.