NEW YORK (Reuters) - Sons of accused Mexican drug lord Joaquin “El Chapo” Guzman murdered a journalist well known for his coverage of drug cartels, a former associate testified at Guzman’s U.S. trial on Wednesday.

FILE PHOTO: Joaquin "El Chapo" Guzman is escorted by soldiers during a presentation in Mexico City, Jan. 8, 2016. REUTERS/Tomas Bravo/File Photo/File Photo

Damaso Lopez Nunez, a former top lieutenant to Guzman, made the accusation in Brooklyn federal court under cross examination by one of Guzman’s lawyers. The lawyer, Eduardo Balarezo, had asked Lopez whether his own son, Damaso Lopez Serrano, might have been involved in the May 2017 murder of Javier Valdez.

Valdez, known for his signature straw hat, was the founder of the RioDoce online newspaper and author of the book “Narcoreporting” about the dangers faced by journalists covering Mexico’s drug war. His murder provoked public outcry about cartel violence.

RioDoce reported last year that Lopez Serrano was likely behind the killing.

Lopez said neither he nor his son had anything to do with Valdez’s murder. He testified that Guzman’s sons killed Valdez for publishing an article about drug cartel infighting against their wishes.

Guzman, 61, was extradited to the United States in 2017 and has been on trial since November on charges of trafficking cocaine, heroin and other drugs into the country as leader of the Sinaloa Cartel. His lawyers have claimed he was framed by another powerful drug trafficker, Ismael “El Mayo” Zambada.

Acccording to media reports, Guzman has fathered as many as 15 children. Lopez did not say which sons murdered Valdez, but he previously testified that four - Ivan, Alfredo, Ovidio and Joaquin - were involved in the cartel.

Lopez, 52, is serving a life sentence in U.S. prison for drug trafficking and has said he is cooperating with prosecutors in the hope of getting the sentence reduced. He has testified that he began working for Guzman in 2001 and is godfather to one of his former boss’ twin daughters.

Earlier on Wednesday, Lopez testified that Guzman’s wife, Emma Coronel, played a key role in plotting her husband’s 2015 escape from a Mexican prison and tried to help him escape again after he was recaptured the following year.

Lopez told jurors that while Guzman was held in Mexico’s Altiplano prison in 2014 and 2015, he plotted his boss’ escape with Coronel and Guzman’s sons, with Coronel passing messages to and from Guzman.

Lopez said the sons bought a plot of land near the prison from which to tunnel into Guzman’s cell. Guzman escaped through the finished tunnel in 2015.

Guzman was recaptured in January 2016. Lopez said Coronel told him the following month that Guzman, then at Altiplano, wanted to duplicate his earlier escape.

That plan was thwarted when Guzman was moved to another prison in Ciudad Juarez, Lopez said, and a $2 million bribe to a national prison official to get him moved back was unsuccessful. Lopez did not identify the bribed official.

Coronel has never been charged with a crime.

When Lopez first stepped up to the witness stand on Tuesday, he looked at Guzman and bumped his fist to his chest. Asked by one of Guzman’s lawyers on cross-examination Wednesday why he made the gesture, Lopez answered, “Because I love him.”

Nonetheless, Lopez said, “the circumstances” compelled him to testify.

“I chose to think about my family,” he said.