The U.S. ambassador to Mexico confirmed Friday that drug lord Ismael "El Mayo" Zambada was brought to the United States against his will when he arrived in Texas in July on a plane along with fellow drug lord Joaquín Guzmán López.

Zambada's attorney had earlier claimed the longtime chief of the Sinaloa cartel had been kidnapped. But officials had not confirmed that, and Zambada's age and apparent ill-health had led some to speculate he turned himself in.

U.S. Ambassador Ken Salazar on Friday said, "the evidence we saw ... is that they had brought El Mayo Zambada against his will."

"This was an operation between cartels, where one turned the other one in," Salazar said. Zambada's faction of the Sinaloa cartel has been engaged in fierce fighting with another faction, led by the sons of imprisoned drug kingpin Joaquín "El Chapo" Guzmán. Guzmán López is the half-brother of the factional leaders.

Salazar said no U.S. personnel, resources or aircraft were involved in the flight on which Guzmán López turned himself in, and that U.S. officials were "surprised" when the two showed up at an airport outside El Paso, Texas on July 25.

Frank Pérez, Zambada's attorney, said in a statement in July that "my client neither surrendered nor negotiated any terms with the U.S. government."

"Joaquín Guzmán López forcibly kidnapped my client," Pérez wrote. "He was ambushed, thrown to the ground, and handcuffed by six men in military uniforms and Joaquin. His legs were tied, and a black bag was placed over his head."

Pérez went on to say that Zambada, 76, was thrown in the back of a pickup truck, forced onto a plane and tied to the seat by Guzmán López.

In early August, Zambada made his second appearance in federal court in Texas after being taken into U.S. custody the week before.

Guzmán López had apparently long been in negotiations with U.S. authorities about possibly turning himself in. Guzmán López, 38, has pleaded not guilty to drug trafficking and other charges in federal court in Chicago.

But U.S. officials said they had almost no warning when Guzmán López's plane landed at an airport near El Paso. Both men were arrested and remain jailed. They are charged in the U.S. with various drug crimes.

Salazar said the plane had taken off from Sinaloa — the Pacific coast state where the cartel is headquartered — and had filed no flight plan. He stressed the pilot wasn't American, nor was the plane.

The implication is that Guzmán López intended to turn himself in and brought Zambada with him to procure more favorable treatment, but his motives remain unclear.

Zambada was thought to be more involved in day-to-day operations of the cartel than his better-known and flashier boss, "El Chapo," who was sentenced to life in prison in the U.S. in 2019.

Zambada is charged in a number of U.S. cases, including in New York and California. Prosecutors brought a new indictment against him in New York in February, describing him as the "principal leader of the criminal enterprise responsible for importing enormous quantities of narcotics into the United States."

The capture of Zambada and Guzmán López — and the idea that one cartel faction had turned in the leader of the other — raised fears that the already divided cartel could descend into a spiral of violent infighting.

That prompted Mexican President Andrés Manuel López Obrador to take the unusual step of issuing a public appeal to drug cartels not to fight each other.