President Andrés Manuel López Obrador of Mexico demanded an apology from President Joe Biden after a ProPublica story disclosed that the Drug Enforcement Administration had found evidence that one of López Obrador’s closest aides accepted donations of around $2 million from a drug cartel in 2006.

In a press conference on Thursday, López Obrador denounced the accusations as “slander” and threatened to curtail U.S.-Mexican cooperation on drug trafficking and immigration issues.

He also accused ProPublica reporter Tim Golden, the author of the article, of being a “pawn” and “a mercenary in the service” of the DEA.

López Obrador said the ProPublica story was part of a media campaign against him by the DEA and the State Department to damage his political party ahead of the presidential election on June 2.

“President Biden should find out about this, because how are we going to be sitting at the table talking about the fight against drugs if they or one of their institutions is leaking information and harming me,” López Obrador said.

“If they have no evidence, they have to apologize,” he said.

López Obrador’s demand comes at an especially sensitive time for the Biden administration, as it seeks to clamp down on the millions of immigrants who have crossed the southern U.S. border in search of asylum. A recent poll showed that immigration has surpassed inflation as the top issue in the presidential race.

The White House, the DEA and the State Department did not respond to requests for comment.

ProPublica published its findings on Tuesday, about the same time that two other news outlets, InSight Crime and Germany’s Deutsche Welle, reported on the DEA investigation.

ProPublica said the article was independently produced and reported, noting that it had repeatedly sought comment from the López Obrador administration before publication.

“We shared a detailed summary of our findings with President López Obrador’s spokesperson more than a week before we published, and he chose not to comment,” said Stephen Engelberg, ProPublica’s editor in chief. “The president has had a lot to say since then, but none of his remarks has identified a single inaccuracy or factual error.”

ProPublica’s article reported that several informants told DEA agents that operatives in López Obrador’s 2006 campaign for president accepted money from the Beltrán Leyva drug mafia. In return, the traffickers were told that a future government would give them a say in naming an attorney general, the informants said. López Obrador did not win that election, but he was voted into office in 2018. As president, he has backed away from confrontation with organized crime groups and scaled back counternarcotics cooperation with the United States.

The DEA investigation — which included testimony from a former campaign operative and a key drug informant — did not establish that López Obrador knew of or approved the donation. However, it found substantial evidence that one of his closest aides had agreed to the arrangement. The investigation was shut down, and no charges were ever filed. Among the reasons, officials in the Department of Justice were concerned that the investigation would be perceived as the United States interfering in Mexico’s politics, ProPublica found.

López Obrador is barred from running for reelection. His party’s candidate is currently leading the race, according to opinion polls. The leading opposition candidate has seized on the accusations.

Golden has written extensively about corruption in Mexico’s government, including a 2022 article about the DEA arrest of former Defense Minister Gen. Salvador Cienfuegos Zepeda. The charges against him were dismissed after Attorney General William Barr decided the case was too politically charged to pursue, ProPublica found.

The DEA has a long history of investigating politicians in other countries for drug trafficking. The Associated Press reported this week that the agency spent years running covert operations in Venezuela to build drug cases against some of its leaders.