The indictment of General Salvador Cienfuegos, a key figure in ex-president Enrique Pena Nieto's 2012 to 2018 government, caught Mexico by surprise.

"Trust and cooperation have been damaged. All protocols between the two countries have been broken," Javier Oliva, a specialist in military affairs at the National Autonomous University of Mexico, told AFP.

The Mexican government said Thursday that it had complained to the United States that it was being kept in the dark about Cienfuegos's case.

"We have made our deep dissatisfaction known to the United States that this information has not been shared with our country," Foreign Minister Marcelo Ebrard told reporters.

Cienfuegos was detained at Los Angeles International Airport on October 15 while on a trip with his family, and charged with drug trafficking and money laundering.

The 72-year-old retired general, nicknamed "The Godfather," is accused of conspiring to produce and distribute cocaine, heroin, methamphetamine and marijuana in the US, according to federal prosecutors in New York.

He allegedly abused his position "to help the H-2 Cartel, an extremely violent Mexican drug trafficking organization," according to documents released by prosecutors.

- 'Resentment' in Mexico -

The indictment was filed in August 2019, but made public only after his arrest, apparently due to suspicion of corruption among high-ranking Mexican officials.

Mexican President Andres Manuel Lopez Obrador, who has good relations with his US counterpart Donald Trump, complained that the US Drug Enforcement Agency (DEA) had acted behind his back.

Mike Vigil, former head of international operations for the DEA, said that since Cienfuegos no longer holds any official position, there was no reason to inform Mexico of his indictment.

But the arrest would have consequences, he said.

"In Mexico there's resentment, and many in the US agencies no longer want to share information with people in the military whom they consider corrupt," Vigil told AFP.

The case is being handled by the same court that sentenced drug lord Joaquin "El Chapo" Guzman to life in prison in 2019 and where Mexico's former public security minister Genaro Garcia Luna is being prosecuted.

Garcia Luna was detained in Texas in December 2019 on charges of taking huge bribes to allow the notorious Sinaloa cartel to ship drugs into the US, which he denies.

- 'Brutal interference' -

At least four people who worked with Cienfuegos are still on active duty, although Lopez Obrador has sought to reassure the country that he personally chose current Defense Minister Luis Sandoval.

"Corruption moves through networks, not individuals. There may be others involved," said Francisco Rivas, head of the National Citizen Observatory, a civil society group.

US prosecutors say the evidence against Cienfuegos includes thousands of Blackberry messages between the general and cartel members intercepted by the authorities.

But some experts are skeptical.

"How is a general with a brilliant career going to give instructions on an open phone to a low-profile criminal?" Oliva said.

The claim of the intercepted messages also fueled longstanding suspicions that US agents spy on Mexican officials.

"It seems to me a brutal interference by the United States," Renato Sales, Mexican National Security Commissioner between 2015 and 2018, told the newspaper El Universal.

He attributed what he called the "unlikely" accusations to rivalry between US agencies ahead of the November 3 presidential election.

Cienfuegos's arrest has triggered speculation over whether US authorities could use him to go after even bigger targets with potentially even greater repercussions for US-Mexican relations.

"Detainees are always asked to turn over people above them. The only one above Cienfuegos was Pena Nieto," said Vigil.