The Justice Department plans to drop drug trafficking and corruption charges against a former Mexican defense minister to allow Mexican officials to investigate him, Attorney General William P. Barr announced Tuesday in an abrupt reversal a month after the official was arrested in Los Angeles.

The official, Gen. Salvador Cienfuegos Zepeda, had been Mexico’s defense minister from 2012 to 2018. Mr. Barr and Mexico’s attorney general, Alejandro Gertz Manero, stopped short in a statement of promising any charges in Mexico, and the move immediately raised questions about why the Justice Department would agree to hand over the matter to a country with a shoddy record of investigating high-profile cases tied to organized crime.

“In recognition of the strong law enforcement partnership between Mexico and the United States, and in the interests of demonstrating our united front against all forms of criminality, the U.S. Department of Justice has made the decision to seek dismissal of the U.S. criminal charges against former Secretary Cienfuegos, so that he may be investigated and, if appropriate, charged, under Mexican law,” Mr. Barr and Mr. Gertz Manero said in the statement.

In a court filing, prosecutors acknowledged that the Trump administration had determined that preserving its relationship with Mexico prevailed over pursuing the case. “The United States has determined that sensitive and important foreign policy considerations outweigh the government’s interest in pursuing the prosecution of the defendant, under the totality of the circumstances, and therefore require dismissal of the case,” they wrote in asking a judge to dismiss the charges.

The explosive accusations against General Cienfuegos had rattled relations between the United States and Mexico, and the quick reversal may be a testament to the close relationship between President Trump and President Andrés Manuel López Obrador of Mexico, who has refused to recognize the electoral victory of President-elect Joseph R. Biden Jr. and has praised Mr. Trump’s treatment of Mexico, saying he “never made a decision without consulting us and always respected us.”

Mr. Barr said that as part of an agreement with the Mexican authorities, the Justice Department had provided the evidence it had collected against General Cienfuegos to investigators there.

The first high-ranking military leader to be arrested on American soil on charges of drug-related corruption, General Cienfuegos was arraigned this month in federal court in Brooklyn, pleading not guilty.

The case made a huge splash in Mexico, dominating the headlines for days as word spread of the extent of the allegations that investigators made against Mr. Cienfuegos.

Among them: Thousands of Blackberry messages coordinating with cartel leaders, orchestrating meetings between military officials and organized crime figures, starting operations against rival traffickers.

The decision was a surprising turnaround for American law enforcement officials, who had accused General Cienfuegos of helping transport narcotics and tipping a drug cartel off to American investigations into their operations.

“That is a pretty stunning turnaround,” said Jim Walden, a former assistant U.S. attorney in Brooklyn. “To bring such high-profile charges and then a month later to defer prosecution to a country where we have seen very mixed results in terms of its criminal justice system — that is an eyebrow raiser.”

Whether or not Mexico has the capacity to conduct such a prosecution remains to be seen. Impunity rates are extraordinarily high for even common crimes — more than 90 percent of homicides go unsolved nationwide.

To take on a top general with tremendous resources and support in a country with such fragile rule of law is likely to be difficult. Mexican authorities have bungled several investigations against drug cartel leaders, including some where American investigators ultimately took over and brought charges in the United States.

Mexico’s anger at the charges stemmed from largely being kept out of the loop on the case, officials have said. Mr. López Obrador himself expressed some surprise at the detention of a military leader who had long commanded respect inside Mexico.

Mexican officials have said privately that they were angry at a lack of communication by Justice Department officials on a case that had clearly taken time to build, given how closely the two countries collaborate in fighting organized crime.

The United States has successfully extradited and prosecuted many drug traffickers from Mexico with the nation’s blessings.

But with Mr. Cienfuegos, there was little of the same cooperation. And in the aftermath of the indictment, officials in Mexico fought back against the accusations. The military, which plays an outsized role in the nation’s domestic security, also stood by its former commander.

The Justice Department charges against General Cienfuegos underscored the corruption that has touched the highest levels of the government in Mexico. General Cienfuegos served as defense minister to President Enrique Peña Nieto, who left office two years ago. And his arrest came 10 months after another top official — who once led the Mexican equivalent of the F.B.I. — was indicted in New York on charges of taking bribes while in office to protect the powerful Sinaloa drug cartel.

Azam Ahmed and Alan Feuer contributed reporting

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