Javier Valdez, 50, was shot near the premises of one of the Mexican news outlets he worked for in the city of Culiacan in the northwestern state, the source said.

Valdez was the fifth journalist to be killed this year in a country plagued by violence related to drug gangs, according to officials and media rights groups.

The attack "was outside the premises of Riodoce... he was shot," the judicial source said.

"Forensic staff are at the scene at this very moment carrying out technical analyses."

Mexico ranks third in the world for the number of journalists killed, after Syria and Afghanistan, according to media rights group Reporters Without Borders (RSF).

Last year, 11 journalists were killed in Mexico overall, it said.

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

One of the most renowned journalists in the state, Valdez had written about the Sinaloa drug cartel and its founder Joaquin "El Chapo" Guzman.

Guzman is currently in jail awaiting trial in the United States.

Valdez worked for the national daily newspaper La Jornada and the local weekly Riodoce. He had also been a local contributor to AFP for more than 10 years.

Last year, he published a book about drug gangs and the media.