Mexico City, Mexico - Mexican President Claudia Sheinbaum told reporters on Wednesday that the Mexican military was unaware of the presence of US agents who were reported to have died in a car accident during an anti-drug operation in the border state of Chihuahua.
"Evidently, the military didn't know there were people participating who weren't Mexican citizens... that there were foreigners participating in the operation," Sheinbaum told reporters.
"This is something that Mexicans shouldn't take lightly."
Sheinbaum said the federal government is still investigating a potential violation of national security laws in the still-murky incident.
Two US embassy employees died in a car accident, the ambassador in Mexico City said on Sunday.
Mexican authorities have given a fuller picture, saying the Americans died alongside two Mexican police and had taken part in a raid on a clandestine drug lab in the northern state's remote sierra.
The five-car convoy included soldiers and members of the Chihuahua State Investigation Agency, authorities said.
The US agents killed were "instructor officers" who "were carrying out training tasks" as part of binational anti-drug cooperation, according to prosecutor Cesar Jauregui.
US media reports have identified the US personnel as CIA agents.
On-the-ground cooperation between US law enforcement and Mexican security forces is historically rare and a politically sensitive topic in the country. Sheinbaum has publicly rejected the practice.
Deadly drug operation fuels tensions between US and Mexico
Home to the key border city of Juarez, Chihuahua is one of only four out of 31 states governed by the conservative National Political Action party, which opposes Sheinbaum's left-leaning Morena party.
The leader said she planned to meet with the Chihuahua state governor to discuss the incident.
Sheinbaum has resisted the Trump administration's threats to use airstrikes or ground troops to fight Mexican cartels, instead promoting intelligence sharing with the US while local security forces attack crime groups.
Elite Mexican troops operating with US intelligence killed drug kingpin Nemesio "El Mencho" Oseguera in February, unleashing a wave of violence that left over 70 dead.