In the spring of 2019, as a new Mexican government shut down most of its cooperation with the United States in the fight against drug trafficking, a small group of American drug agents decided to confront the problem in a different way.

Sifting through databases and court files, they compiled dossiers on Mexican officials suspected of colluding with the mafias. Months later, federal prosecutors used the evidence to indict a former security minister, Genaro García Luna, the most important Mexican figure ever convicted on U.S. drug corruption charges.

The senior agent who led the team, Terrance C. Cole, was not rewarded for his efforts. He sought a promotion to run the Drug Enforcement Administration’s Mexico City office but was passed over. Frustrated with the agency’s direction and his own career trajectory, he retired in 2020 to take a job with a software company before becoming Virginia’s secretary of public safety in 2023.

Five years later, Cole is returning to run the DEA, having emerged as President Donald Trump’s unexpected choice for the position.

Unlike other former agents who have led the DEA, Cole never rose to its top ranks or even ran one of its 23 domestic field divisions. His most significant leadership experience has been overseeing police, prisons and emergency response agencies under Virginia Gov. Glenn Youngkin, a Trump ally who championed Cole for the DEA post.

But with the White House promising an all-out fight against the traffickers who have flooded U.S. markets with fentanyl and other illegal drugs, Cole would bring an unusual background to the job. That includes some searing experiences with the corruption that sustains the drug trade, and a conviction that the United States cannot successfully fight the traffickers without also taking on the officials who abet their operations.

“The Mexican drug cartels work hand-in-hand with corrupt Mexican government officials at high levels,” Cole said in an interview with the far-right news site Breitbart shortly after his retirement. “If the average taxpayer had a basic understanding of how these two groups work together still — to this minute — they would be sickened.”

The Trump administration has warned that it is prepared to take unilateral actions against drug mafias in Mexico if the government there does not greatly escalate its own efforts. But current and former officials said White House discussions have been more focused on the tactics it could use against the traffickers — from drone strikes to cyber operations — than on any longer-term strategy to weaken them.

The administration may also have set in motion a new era of interagency competition on the issue, with the CIA and the Defense Department presenting proposals to expand U.S. intelligence collection on traffickers in Mexico and try to disrupt their operations in ways that may or may not complement the efforts of the DEA and other law enforcement agencies.

How U.S. officials might confront Mexico’s endemic corruption remains an open question. But after decades in which the problem has been mostly subordinated to other U.S. interests, it is likely to command a higher priority in American policy — and to unsettle the U.S. relationship with Mexico.

In its first announcement of punitive tariffs on Mexico, the White House cited “an intolerable alliance” between the government and the drug trade. “This alliance endangers the national security of the United States, and we must eradicate the influence of these dangerous cartels,” it said.

Hoping to avoid an economic calamity, Mexico has conspicuously intensified its own drug enforcement efforts since then. But when asked about Cole’s nomination, President Claudia Sheinbaum warned that she would uphold the sharp restrictions on DEA activities in Mexico imposed by her predecessor, Andrés Manuel López Obrador.

“We will never permit interventionism or violations of our sovereignty,” Sheinbaum said. “It will not be like before President López Obrador, no.”

Privately, some DEA veterans have lobbied against Cole. Those former officials, most of them associated with the agency’s Special Operations Division, have questioned Cole’s qualifications for the job in discussions with Senate staff aides, but they have been unwilling to air their criticism publicly.

A former college lacrosse player, Cole was described by colleagues as a driven, competitive and sometimes abrasive agent and supervisor. As a rookie agent in McAlester, Oklahoma, Cole made enough of an impression to be sent in 2002 to Bogotá, Colombia, in the early years of the billion-dollar U.S. aid program known as Plan Colombia.

The ambitious U.S. effort sought to help Colombia transform its criminal justice system, root out corruption, and combat the interwoven threats of drug gangs, leftist guerrillas and right-wing paramilitary groups. At the center of the plan was the creation of elite police teams, vetted and trained by the DEA, that operated alongside U.S. intelligence and law enforcement agencies.

The team that worked with Cole and several other agents was among Colombia’s most effective, former DEA officials said. In Bogotá, it made a series of arrests and drug seizures that struck at the Norte del Valle Cartel and its leader, Diego Montoya. It also uncovered evidence that the cartel had co-opted high-level officials in both the police and military, they said.

“We were doing amazing things,” Cole recalled last year on a podcast with Republican former U.S. Rep. Mary Bono. “Working some of the biggest corruption cases, against some of the highest-level Colombian government officials. But on May 22, 2006, that’s when it all came crashing down for me.”

That day, an informant walked into the Colombian team’s offices in Cali offering a tip that Montoya’s men had stashed some cocaine in the nearby town of Jamundí. After seeking approval from senior police officials but not the DEA, agency officials said, the team leader gathered nine of his agents and drove off with the informant to investigate.

As they pulled up to the isolated location, the police came under a barrage of gunfire. The shooting continued for 20 minutes until all 10 agents and their informant were dead. When Cole arrived at the scene that night with the Colombian attorney general and the head of the national police, they found the agents’ bodies on the ground; the Colombian army soldiers who attacked them were still on the hillside above them.

Cole was devastated.

“Those guys worked very closely with him,” his supervisor, Matthew Donahue, said. “We depended on them, and they depended on us. It was like having your partner killed.”

Although the army claimed that the shootings were a tragic accident, the attorney general found that the informant had been planted by the traffickers and that the lieutenant colonel who led the troops had organized the ambush. In 2008, he and 14 soldiers were convicted of aggravated homicide.

A few months after the killings, Cole went ahead with a planned tour of duty in Afghanistan. There, he found again that U.S. allies in the war were sometimes as involved in the drug trade as the Taliban insurgents they fought.

In 2008, Cole moved to Dallas, where he earned a reputation as a sharp-elbowed group supervisor who pushed his agents to get their photographs on the office wall by making the biggest cases and seizing the biggest loads. He was regarded highly by his superiors, several former colleagues said, but less popular with some of his peers.

By 2010, Cole’s squad was focused on the Texas distribution network of the Zetas, then widely seen as the most violent of Mexico’s drug mafias, and one of its leaders, Miguel Treviño Morales.

By leveraging the cooperation of traffickers facing prosecution, one of Cole’s agents obtained a list of cellphone numbers being used by Treviño; his brother, Omar; and their lieutenants. It was a coup — a way to perhaps intercept the Zeta leaders’ calls and encrypted text messages or even track their movements in real time.

On March 9, 2011, government records show, Cole entered the eight numbers and a PIN code for one of the phones into a secure agency database. He then forwarded them to the DEA’s Special Operations Division, which could sometimes intercept or geolocate cellphones overseas with the help of U.S. intelligence agencies.

Cole also sent the numbers to the DEA offices in Mexico City and Nuevo Laredo, where other agents were investigating the Zetas, officials said. Ten days later, gunmen led by the Treviño brothers roared into the Mexican border town of Allende, where the DEA’s informants had been operating. The traffickers began torturing and murdering anyone who they suspected might be connected to the men they thought had betrayed them, killing as many as 200 men, women and children.

In a 2017 article, ProPublica reported that Cole’s forwarding of the numbers to U.S. agents in Mexico — who then shared them with a DEA-trained Mexican police unit that warned the Zetas — led to the Treviños’ rampage. Only years later did the DEA, prodded by Congress, even review its files on the case; it never investigated its possible role in the massacre.

Cole declined to be interviewed for ProPublica’s article, and a White House spokesperson said he could not comment on the case now because the Treviño brothers, who were handed over to the United States by Mexico on Feb. 27, are facing prosecution for trafficking, murder and other crimes. They pleaded not guilty last month in a Washington, D.C., federal court.

The White House spokesperson said “of course” Cole and other DEA officials considered the sensitivity of sending the Zetas’ phone information to Mexico but followed standard protocols in doing so. A former deputy head of the DEA office in Dallas, Daniel Salter, said he and the special agent in charge there made that decision, not Cole.

At least three senior Mexican police officials who might have had access to the phone numbers shared by the DEA have since been charged in the United States with colluding with the traffickers. But officials said that subsequent DEA reporting also pointed to another reason why the Treviños might have turned on the informant who was their primary target in Allende: He owed them some $30 million and was blamed for some earlier U.S. seizures of drugs and cash.

After Dallas, Cole spent four years at the agency’s Washington-area headquarters, watching as U.S. law-enforcement agencies struggled with the Mexicans to hunt down well-protected drug bosses, like Joaquín “El Chapo” Guzmán, without making any substantial impact on the flow of drugs.

But even that halting cooperation came to an end as Mexico’s new president, López Obrador, took office promising to fight the drug trade with “hugs, not bullets.” He sidelined police teams trained by the DEA, shut down a Mexican marine commando unit that had been the country’s most effective weapon against the traffickers and even refused to grant visas to DEA agents assigned to Mexico.

Former officials said Cole, who arrived in Mexico City in late 2018 as a deputy director of the DEA’s regional office, soon proposed a radical solution: If the agents couldn’t get Mexican officials to work with them to pursue the traffickers, what about going after the corrupt officials who were protecting the traffickers’ operations?

For decades, U.S. investigators had generally avoided such targets, lest they be seen as interfering in internal Mexican politics. But the extradition of high-level Mexican traffickers over the previous decade had created a pool of criminals eager to reduce their sentences by helping U.S. prosecutors, and many were willing to testify about the officials they had bribed.

A team of DEA agents pulled together files on some 35 possible targets, ranging from police and military commanders to Mexican cabinet officials. One target that stood out was García Luna, the once-powerful security minister who had worked closely with U.S. officials.

While the Biden administration hailed García Luna’s prosecution in 2023 as proof of its mettle in pursuing corruption, it also worked assiduously to avoid drug enforcement actions that might antagonize López Obrador and jeopardize his help in controlling illegal migration.

Cole was by then gone from the DEA, having left Mexico City after just a year. He had once hoped to succeed Donahue there but was not seriously considered for the post. He retired from the agency after 22 years.

As Virginia’s secretary of public safety and homeland security, Cole focused on trying to limit fentanyl trafficking, an effort that drew the attention of Trump supporters. While he kept a fairly low public profile, Cole’s tough rhetoric on Mexico was also very much in line with Trump’s.

“Mexico has been a failing state for years,” he told Bono. Referring to the reported recruitment of foreign mercenaries by the drug gangs, he added, “Now we’re seeing Mexico turn into a terror training camp similar to what we saw in the Middle East years ago.”

Although the Trump administration’s attention to the drug issue has raised the DEA’s profile, Cole will, if confirmed as the administrator, likely have to fight for its place in a growing bureaucratic scrum.

Already, officials said, the FBI and Homeland Security Investigations have been pushing to lead the Trump administration’s campaign against trafficking groups that it has designated as terrorist organizations. The CIA and the Defense Department have also expanded their efforts to collect intelligence on the traffickers and put forward options for more aggressive actions to strike at their operations.

With Sheinbaum still attacking the DEA as a symbol of American interventionism, all four of those competing agencies may have an easier time rebuilding trust with the Mexican government. But while Mexican leaders insist they will act on hard evidence of corruption in their ranks, many U.S. officials remain skeptical that they will be able to make a serious push for such action without upending the two countries’ relationship.