Two U.S. Embassy staffers and two Mexican law enforcement officials died in an unspecified accident following a drug eradication operation in Mexico, authorities said Sunday.

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The four were returning from an operation to destroy drug labs in the municipality of Morelos in the state of Chihuahua when the accident took place, Chihuahua Attorney General César Jáuregui Moreno said at a news conference.

The U.S. staffers, described as instructors who train and collaborate with Mexican law enforcement, were not identified.

In a statement, U.S. Ambassador to Mexico Ronald Johnson called the deaths a “tragic loss” and said they are “a solemn reminder of the risks faced by Mexican and U.S. officials dedicated to protecting our communities.”

Jáuregui identified the Mexican law enforcement personnel as State Investigation Agency Director Pedro Román Oseguera Cervantes and Officer Manuel Genaro Méndez Montes, also of the agency.

“We extend our deepest condolences and our sympathies to the families of those who died in this tragic accident,” he said.

Maru Campos, governor of Chihuahua, praised Oseguera for “working for the peace and security of the people of Chihuahua.”

Morelos is about 360 miles southwest of the U.S.-Mexico border at the Good Neighbor Bridge crossing in El Paso, Texas.