Mexican drug lord Ismael “El Mayo” Zambada and the son of his former partner, Joaquin “El Chapo” Guzman, were arrested on Thursday in El Paso, Texas, in a major coup for US authorities that may also reshape the Mexican criminal landscape.

Zambada is one of the most consequential traffickers in Mexico’s history and co-founded the Sinaloa Cartel with El Chapo, who was extradited to the United States in 2017 and is serving a life sentence in a maximum security prison.

Both Zambada and Joaquin Guzman Lopez, the son of El Chapo, face multiple charges in the US for funnelling huge quantities of drugs to US streets, including fentanyl, which has surged to become the leading cause of death for Americans between the ages of 18 and 45.

Zambada, who is believed to be in his 70s, and Guzman Lopez, who is in his 30s, were detained after landing in a private plane in El Paso, two US officials told Reuters.

Guzman Lopez is one of four sons of El Chapo known as Los Chapitos, or Little Chapos, who inherited their father’s faction of the Sinaloa Cartel.

His brother, Ovidio Guzman, was arrested last year and extradited to the United States.

In recent years, the Sinaloa Cartel has become the biggest target for US authorities, who have accused the crime syndicate of being the biggest supplier of fentanyl to the United States.

Zambada and Guzman Lopez face multiple charges in the US “for heading the Cartel’s criminal operations, including its deadly fentanyl manufacturing and trafficking networks,” US Attorney General Merrick Garland said in a statement.

The arrest of Guzman Lopez was first reported by Reuters, ahead of the Justice Department statement where it was confirmed they were detained in El Paso.

The US authorities had a $15 million reward for Zambada’s capture, while Guzman Lopez had a $5 million bounty on his head.

The Sinaloa cartel traffics drugs to more than 50 countries around the globe and is one of the two most powerful organized crime groups in Mexico, according to US authorities.

Zambada and El Chapo’s sons belong to two different generations of traffickers, with differing styles.

Zambada is known for being an “old-school” narco, avoiding the limelight and operating in the shadows.

El Chapo’s sons, by contrast, have a reputation for being flashy narcos who courted attention as they ascended the rank of the cartel.

El Chapo’s sons are also known to be more violent and hot-headed than Zambada, who had a reputation as a shrewd operator.

Zambada and El Chapo’s sons have had a fractious relationship since their father was extradited in 2017, and the arrests of Zambada and Guzman Lopez may trigger instability or even violence in their heartlands in the northern state of Sinaloa.

Their arrests were part of a joint operation by the Federal Bureau of Investigation (FBI) and Homeland Security Investigations (HSI) agencies, two US officials said.

US authorities have over the past year launched fresh indictments against Zambada and Guzman’s sons on new charges in the United States that focus on fentanyl smuggling, as well as the flow of precursor chemicals to the illicit labs operated by their crime syndicate.

Over decades, the cartel has set up sophisticated supply chains to move drugs across the globe and to source heavily regulated chemicals to their home base in Sinaloa.

Earlier in the day on Thursday, Reuters published an investigation that revealed how fentanyl precursor chemicals are astonishingly easy and cheap to buy online from Chinese sellers that ship the substances door-to-door in North America.

“Fentanyl is the deadliest drug threat our country has ever faced, and the Justice Department will not rest until every single cartel leader, member, and associate responsible for poisoning our communities is held accountable,” Garland added.