US authorities have detained the defence minister for Mexican ex-president Enrique Pena Nieto at Los Angeles airport on a US Drug Enforcement Administration (DEA) warrant, a DEA spokeswoman says.
As defence minister General Salvador Cienfuegos was a powerful figure in Mexico's drug war in which the army battled cartels across the country.
Several of Mexico's former top-ranking 'drug war' officials have been implicated in narcotics.
The high-level arrest comes less than three weeks before the US presidential election.
President Donald Trump, seeking a second term, has made clamping down on cartel activity a major policy objective, though with little major progress since he took office in 2017.
Mexico's Foreign Minister Marcelo Ebrard said on Twitter that he was informed of the detention of Cienfuegos by the US ambassador to Mexico.
"The Consul in Los Angeles will be informing me of the charges in the next few hours. We will offer the consular assistance to which he is entitled. I will keep you posted," said Ebrard.
A Mexican diplomatic source said members of Cienfuegos' family, travelling with him at the time of the detention, have already been released.
The DEA spokeswoman told Reuters she did not have further details on the circumstances of the arrest, whether Cienfuegos was coming or going at the Los Angeles airport, or whether family or others were travelling with him.
"This detention is going to have a powerful impact in Mexico," said military affairs analyst Raul Benitez.
Cienfuegos was Pena Nieto's defence minister until the end of the president's term.
Pena Nieto was in office for six years through December 2018. Several members of his cabinet and party have been implicated in high-level corruption cases, sometimes involving allegations of links to organised crime.
Under Cienfuegos the army was accused of extrajudicial killings, including the 2014 Tlatlaya massacre, where 22 drug gang members were executed.
Arrest warrants were issued in Mexico last month for soldiers in connection with the kidnapping and presumed massacre of 43 students the same year in a town known for heroin trafficking.
Elements of the Army in the northern state of Sinaloa have long been rumoured to have had links to the cartel formerly headed by Joaquin "El Chapo" Guzman, now in a US prison.
The detention of Cienfuegos comes on the heels of the arrest of ex-President Felipe Calderon's security minister, Genaro Garcia Luna.
Garcia Luna is on trial in New York on suspicion of accepting millions of dollars in bribes from Guzman's Sinaloa cartel, which he was tasked with combating.
President Andres Manuel Lopez Obrador has leant heavily on the armed forces in his efforts to end cartel violence in the country, which saw nearly 35,000 killings last year.