Infamous drug lord Rafael Caro Quintero, who was behind the killing of a US DEA agent in 1985, has been captured by Mexican forces nearly a decade after walking out of a Mexican prison and returning to drug trafficking, according to the Mexican government's national arrest registry.

An official with Mexico's navy, who was not authorised to speak publicly and agreed to confirm the action only if not quoted by name, also confirmed the arrest.

The registry listed the time of Caro Quintero's arrest as around midday and said he was in transit, but no further details about the capture were immediately available.

Caro Quintero walked free in 2013 after 28 years in prison when a court overturned his 40-year sentence for the 1985 kidnapping and killing of US Drug Enforcement Agency (DEA) agent Enrique "Kiki" Camarena.

The brutal murder marked a low point in US-Mexico relations.

The Supreme Court later upheld the sentence, but it was too late by then: Caro Quintero had been spirited off in a waiting vehicle.

Caro Quintero, the former leader of the Guadalajara cartel, had since returned to drug trafficking and unleashed bloody turf battles in the northern Mexico border state of Sonora.

Hours after Caro Quintero's arrest, a Mexican navy helicopter crashed near Los Mochis, Sinaloa, killing 14 people on board, said Rear Admiral Jose Orozco, who leads the navy's press office.

"There is not evidence that it was downed, nor that Caro Quintero was aboard," Rear Admiral Orozco said.

He did not clarify whether the helicopter was involved in the operation to arrest Caro Quintero, but local media reported that the capture occurred in that region of north-west Mexico.

Tense relations improve between Mexican government and DEA

President Andrés Manuel López Obrador has maintained that he is not interested in detaining drug lords and prefers to avoid violence. 

But the arrest came just days after López Obrador met with US President Joe Biden in the White House.

There had been tensions between the Mexican government and the DEA after Mexico enacted a law limiting the US agency's operations.

But recently, the DEA's new head in Mexico received a visa, which the US officials marked as a sign of progress in the relationship.

Shortly before Caro Quintero's arrest on Friday, US ambassador Ken Salazar told a gathering of reporters there had been progress in the security relationship.

"I have been in meetings with the foreign minister and with the security cabinet, along with all our agencies that have included the new head of the DEA sitting at my right hand," Mr Salazar said.

Quintero was on the FBI's most wanted list, with a $20 million reward for his capture through the State Department's Narcotics Rewards Program.

He was added to the FBI's top 10 most wanted list in 2018.

Caro Quintero was one of the primary suppliers of heroin, cocaine, and marijuana to the United States in the late 1970s.

He blamed Special Agent Camarena for a raid on a marijuana plantation in 1984.

In 1985, Special Agent Camarena was kidnapped in Guadalajara, allegedly on orders from Caro Quintero. His tortured body was found a month later.

Mike Vigil, the DEA's former chief of international operations, said Caro Quintero was believed to have been operating independently most recently, though there had been rumours he was back with the Sinaloa cartel. 

Caro Quintero was from Badiraguato, Sinaloa, the same area as Joaquín "El Chapo" Guzmán, the former Sinaloa cartel leader now serving a life sentence in the United States.

He eventually became one of the "godfathers" of Mexican drug trafficking.

'The conflict doesn't end'

Mr Vigil called Caro Quintero's arrest a surprise, considering López Obrador's stated disinterest in going after drug cartel leaders, but he added that the DEA would never stop looking for someone who killed an agent.

After seeing Caro Quintero walk out of a Mexican prison once already, Mr Vigil said he hoped the government would not risk a repeat — Guzman twice escaped from Mexican maximum security prisons — and would agree to a likely US extradition request.

In Sonora, one of the states hit hardest by Caro Quintero's efforts to reclaim his territory, there was a hope his arrest could help.

"I believe in Sonora, in general, there could be calm, and yes, relief for us, because I believe the disappearances will diminish," said Cecilia Duarte, an activist with a team of volunteer searchers in Sonora who look for the clandestine graves of the disappeared.

Some activists have been threatened and even killed in Sonora amid Caro Quintero's turf wars with the sons of El Chapo.

But, Mr Duarte said, Caro Quintero "is only part (of the conflict), the conflict doesn't end."