TIJUANA, Mexico - Gunmen ambushed and killed a founder and editor of a crusading Mexican newspaper Tuesday, the latest in a series of attacks against the weekly's leadership.

Masked men in a pickup truck opened fire on Francisco Ortiz Franco as he left a clinic with his two children, the state attorney general's office said. The children, aged 8 and 10, were unharmed.

Zeta is known for its reporting on the influence of drug traffickers in Tijuana, home to several narcotics operations. Its editorial board demanded "an investigation that leads to the killers' capture."

State investigators re-enacting the slaying at the scene said a gunman jumped from a black Jeep and shot Ortiz four times as he sat in his driver's seat, according to Francisco Castro Trenti of the attorney general's office.

Virginia Monje, 30, said she heard the shots from her kitchen. She went outside to see Ortiz's two young children running from the scene shouting "Papi! Papi!"

The newspaper has been targeted before. Its co-founder, Hector Felix Miranda, was ambushed and killed on April 20, 1988. Two men were convicted in the shooting.

In 1997, the newspaper publisher Jesus Blancornelas was badly wounded in a gangland-style attack that killed his bodyguard.

Shortly before he was shot, Blancornelas had written a column linking the Arellano Felix drug gang for the shooting deaths of two Mexican federal agents.

Mexican President Vicente Fox ordered federal officials to collaborate with local authorities in the investigation.

"The federal government reiterates its condemnation of every act that pretends to weaken the integrity of journalists and its conviction that a free and critical press is the best guarantee for the strengthening of our democracy," the release said.

In Mexico City, federal Attorney General Ramon Macedo de la Concha said he had no firm details about the incident, but told reporters "If we have evidence that it deals with organized crime, we will take over the case."

After Blancornelas was shot, Ortiz vowed that threats would not deter the staff. Ortiz, one of three editors at the newspaper, specialized in legal affairs.

"Obviously, we are not going to change," he said. "We are used to working on deep investigations, and that's not going to change."

Ortiz - who trained as an attorney and was a co-founder of the newspaper that emerged in 1980 - was among several journalists and government officials working with a Mexican advisory task force and the Miami-based Inter-American Press Association on an investigation into Felix Miranda's murder.

The association, or IAPA, issued a statement Tuesday afternoon expressing its "outrage" at Ortiz's killing. IAPA official Rafael Molina called for a rapid resolution.

"Yet another crime in Tijuana must not go unpunished," Molina said.

The attack highlighted the risks journalists throughout Latin America, including Mexico, have long faced, especially when their work targets government corruption or drug trafficking.

Roberto Javier Mora Garcia, executive editor of El Manana newspaper in the northern border state of Nuevo Laredo, was found stabbed more than 25 outside his home in March. He had often published front-page stories on the Gulf drug cartel.