Mexican President Claudia Sheinbaum has ordered an investigation into the role played by two US officials in a counter-narcotics operation in the northern state of Chihuahua.
The two died alongside two Mexican officials when their car crashed on their way back from an operation to destroy illegal drug labs, Chihuahua officials said.
Sheinbaum stated that neither she nor senior members of the federal security team had been informed about any joint US-Mexican operations.
The Mexican leader has been adamant that foreign officials can only operate on Mexican soil if given prior clearance at the federal level.
Sheinbaum has come under pressure from her US counterpart, Donald Trump, to do more to stem the flow of drugs from Mexico to the United States but she has insisted that Mexico's sovereignty cannot be breached.
On Monday, Sheinbaum said we did not have knowledge of any direct work between Chihuahua state and personnel from the US embassy.
She also stated the government needed to understand the circumstances under which this was taking place, and then assess the legal implications.
According to a Chihuahua state official, the two US nationals and two members of the Chihuahua State Investigation Agency (AEI) died on Sunday morning when the car they were traveling in skidded off the road and fell into a ravine, resulting in an explosion.
The US ambassador in Mexico, Ronald Johnson, described the two American citizens as US embassy personnel.
Chihuahua State Attorney-General César Jáuregui said in a news conference that the two were instructor officers from the US embassy involved in training work as part of the bilateral collaboration between US and Mexican authorities.
He added that the accident happened as they were driving back from an operation in which a number of clandestine labs for the production of synthetic drugs were destroyed. Quizzed again on Monday about the role of the two US officials, he said they were primarily engaged in basic training work, hours away from the drug lab operation.
Sheinbaum stated that Mexican officials had reached out to both the US embassy and Chihuahua state authorities for information to determine if the operation may have breached Mexican national security law, which prohibits joint operations without prior federal approval.
She stressed that while her government collaborates with the US, including sharing intelligence, there are no joint operations on land or in the air.
On Tuesday, the Washington Post reported that the two US officials who died worked for the CIA, which has expanded its role in combating narcotics trafficking in the Western Hemisphere.
The BBC has contacted the CIA for comment.
Last September, a Reuters investigation found that the agency had been running covert operations in Mexico for years to hunt down the country's most-wanted drug traffickers, working closely with special narco-hunting units within the Mexican military.
With the Mexican government's approval, the CIA has provided select Mexican units with training, equipment, and financial support for operations, including travel.
At least two CIA-vetted military units are currently active, including the Mexican Army group that captured Ovidio Guzmán-López and a specialized Mexican Navy intelligence outfit.
