Javier Valdez was killed when assailants opened fire on his vehicle in the state capital, Culiacan, according to RioDoce, the media outlet he co-founded and where he worked.

The official was not authorized to discuss the matter publicly and spoke on condition of anonymity.

Mexican and foreign journalists paid homage to Valdez on social media, describing him as a courageous writer and generous friend whose killers must be brought to justice to deter future slayings.

Cárdenas began his career in journalism in the early 1990s, according to La Jornada, a Mexican news outlet where he worked as a national correspondent.

The CPJ awarded Valdez its International Press Freedom Award in 2011 for his prolific coverage of drug trafficking and organised crime.

He was the fifth journalist (link in Spanish) to be killed in the country so far this year. He was also the author of various books that dealt with the topic of drugtrafficking like "Huerfanos del Narco" (Narco Orphans), "Miss Narco", "Los Morros del Narco" and "Narcoperiodismo", where he talked about the danger of journalists that denounced criminals. President Enrique Pena Nieto condemned what he called an "outrageous crime". The latter chronicled the lives of young people swept up in Mexico's underworld.

Valdez's brother Rafael said the reporter had been "very happy" in recent days and had not indicated that he had received threats. "I don't want to be asked, 'What were you doing in the face of so much death ... why didn't you say what was going on?'"

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The US-based Committee to Protect Journalists (CPJ) this month said "Mexico's press is caught in a deadly cycle of violence and impunity".

Journalists targeted in Mexico are most often local reporters in places where the rule of law is tenuous, but there have also been killings of journalists with national profiles such as Valdez and Regina Martinez Perez, who was slain in 2012.

"Being a journalist is like being on a black list", Valdez said at a launch of his last book about drug gangs.

By the group's count, some 40 journalists have been killed in Mexico for reasons confirmed as related to their work since 1992.

Last Wednesday, the federal Attorney General's Office replaced the head of its division responsible for investigating journalist killings.

An award-winning local journalist and Agence France-Presse contributor who reported on violent drug gangs in Mexico was shot dead in the street Monday, a court source told AFP.