SAN DIEGO (AP) — A Sinaloa cartel leader and his son have been indicted on drug smuggling charges in the United States in what U.S. authorities called a blow to one of Mexico’s most powerful drug rings.

The indictment against Damaso Lopez Nunez was unsealed in Virginia as his 29-year-old son made his initial court appearance in San Diego. The son, Damaso Lopez Serrano, turned himself in to U.S. border inspectors in California less than two weeks ago.

Damaso Lopez Nunez has been battling for control of the Sinaloa cartel following last year’s arrest of billionaire kingpin Joaquin “El Chapo” Guzman, who faces drug charges in the United States.

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Known by the nickname “El Licenciado,” a title for college graduates, Lopez was long considered Guzman’s right-hand man and helped him escape from a Mexican prison in 2001. Mexican authorities arrested him in Mexico City in May, and U.S. authorities are seeking his extradition.

The younger Lopez, known as “Mini Lic,” surrendered to U.S. Customs and Border Protection officers July 27 in Calexico, east of San Diego, authorities said. He entered a plea of not guilty Monday at a hearing held under tight security and was ordered to be held without bail. He is believed to be the highest-ranking Mexican cartel member ever to surrender to authorities in the United States.

David Shirk, an associate political science professor at University of San Diego, said it was highly unusual for a cartel operative to turn himself in to U.S. authorities and that it may be another sign that the Sinaloa cartel is “under siege.”

“The fact that a member of this organization is surrendering to U.S. authorities shows that something is going on and that something may be going wrong for the Sinaloa cartel,” Shirk said.

U.S. Attorney General Jeff Sessions said the indictments show that the Justice Department has made going after Mexican cartel leaders a top priority.

“We will continue to go after these cartel leaders in order to dismantle their organizations from top to bottom, and today’s announcement should send them a clear message: you can turn yourselves in the easy way, or we will find you and bring you to justice the hard way,” Sessions said in a statement.

The elder Lopez is believed to have been locked in a dispute with Guzman’s sons for control of the cartel’s territories. The head of Mexico’s federal detectives’ agency, Omar Garcia Harfuch, said in May that Lopez was “one of the main instigators of violence” in the Mexican states of Sinaloa and the southern part of Baja California.

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The father and son are named in the indictment in the Eastern District of Virginia, which was filed in December. They are charged with conspiracy to import and distribute controlled substances.

The San Diego indictment, which was filed in August 2016 and ordered unsealed Monday, charges Lopez Serrano with conspiracy to import and distribute methamphetamine, cocaine and heroin. Also charged was Nahum Abraham Sicairos Montalvo, who was arrested last month near Mexico City and identified by Mexican authorities as a major financial operator for the Sinaloa cartel.

Adam Braverman, an assistant U.S. attorney in San Diego, said the investigation lasted five years and involved intercepts of more than 250 communication devices. In 2015, there were tens of thousands of communication intercepts and “a number of (drug) seizures” in Mexico and the United States.

Lopez Serrano appeared in an orange uniform for jail inmates and said little in court.

His attorney, Michael Littman, entered the not-guilty plea on his client’s behalf. Littman declined to comment as he left the courtroom.

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Associated Press writer Julie Watson contributed to this report.