At a news conference this week, Mexico’s attorney general, Alejandro Hertz, said López Serrano was the “instigator” of Valdez’s murder.
“We have already brought the other culprits to justice, they are in prison,” he said.
Mr Hertz added that Mexico had called for his extradition “on countless occasions” but had been refused because US authorities considered López Serrano a “protected witness” who was “giving them a lot of information”.
Investigators believe López Serrano ordered Valdez’s killing after he was angered by journalists’ coverage of an internal power struggle within the Sinaloa cartel.
López Serrano’s father, Damas López Núñez, was considered a key lieutenant of cartel boss Joaquin “El Chapo” Guzmán.
After Guzmán’s arrest and extradition to the United States, López Núñez began a bloody struggle for control of the cartel, but was eventually captured in a raid in Mexico City in 2017.
In July 2017, López Serrano surrendered to US authorities to face drug-trafficking charges and cooperated in exchange for a reduced sentence.
At the time, U.S. law enforcement officials described him as a “high-ranking Mexican cartel leader” who had “self-made” the U.S.
He was released from prison on parole in 2022. He was re-arrested Friday to face additional fentanyl trafficking charges.
At the time of his death, Valdés was covering a bloody power struggle within the Sinaloa cartel pitting López Núñez and López Serrano against the Guzmán sons.
Eight days before his death, he published a column in which he described López Serrano as spoiled, “good for a chat but not for business” and “a weekend fighter with a dummy gun.”
Mexico is one of the harshest countries in the world for journalists.
According to Reporters Without Borders, more than 150 journalists have been killed there since 1994.
At least 15 people were killed in 2022, making it one of the most brutal years for Mexican journalists.
The violence continued. In October, a journalist was shot dead in the violence-ridden town of Uruapan.
The next day, a female reporter was killed in a restaurant she owned in Kalima State.
