A reputed top assassin for the Arellano Felix drug cartel was arrested after a shootout with police in Tijuana in connection with Tuesday's killing of a prominent local journalist, Mexico's attorney general said.

The arrest on Thursday of Mario Alberto Rivera Lopez, alias "El Cris," came as President Vicente Fox promised an aggressive investigation. Dozens of journalists marched in Tijuana to protest the slaying of Francisco Ortiz Franco, an editor at the weekly newspaper Zeta. Ortiz was shot to death in front of his two young sons.

While the arrest was welcomed by residents in Tijuana, it did little to resolve widespread concern in the border city, just south of San Diego, which remains a violent haven for drug traffickers who often seem more powerful than the authorities trying to capture them.

"The fact is that Tijuana continues to be a killing field for those who challenge the powers that be," said Sergio Aguayo, an academic and human rights advocate in Mexico City. "There are still some cities and regions in Mexico that are outside the legal system and completely out of control."

Victor Clark Alfaro, a human rights activist in Tijuana, said the only thing unusual about Ortiz's death was that it made headlines. Clark said that Tijuana struggles with drug-related murders almost every day: "The only thing for sure is that tomorrow there will be another body, of someone totally unknown."

Rafael Macedo de la Concha, Mexico's attorney general, said Rivera was not charged with the Ortiz killing, but said he believed Rivera and those arrested with him were responsible because the "manner in which these assassins operate" was similar to the way in which Ortiz was killed.

The arrest of Rivera and three other reputed drug gang members also illustrates the danger of the Arellano Felix drug cartel, even after the loss of its top leaders. Cartel boss Benjamin Arellano Felix was arrested in March 2002. A month before that, his brother Ramon, the cartel's feared enforcer, was killed in a shootout. This month, two top leaders who took over from the brothers were arrested.

Macedo said that when the alleged gunmen were surprised by federal agents in a house in the Loma Dorada neighborhood of Tijuana, they fired on the agents with a 50mm military-style machine gun.

In the edition of Zeta published Friday, editor Jesus Blancornelas, who was severely wounded in an attack by Arellano Felix gunmen in 1997, wrote that he believes there are three main suspects in Ortiz's killing, including the cartel's assassins and Los Zetas, a group of former Mexican army soldiers now working with drug traffickers.

Blancornelas wrote that he also suspects the involvement of Jorge Hank Rhon, the son of one of Mexico's most powerful politicians and currently a Tijuana mayoral candidate. Blancornelas and Hank Rhon have feuded since 1988, when security guards who worked for Hank Rhon were convicted of killing Zeta's founding editor, Hector "El Gato" Felix Miranda. In every issue since, Zeta has run a full-page ad asking Hank Rhon why his bodyguards killed Felix Miranda. Ortiz had been involved in a new investigation of the case by the Mexican government and the Inter American Press Association. Hank Rhon has repeatedly denied any role in the killing.