MEXICO CITY — A veteran journalist who had chronicled the bloody conflicts among rival drug cartels in his home state, Sinaloa, and the culture of violence they inflicted on the broader society, was killed by gunmen on Monday near the newspaper that he had co-founded, the authorities said.

The journalist, Javier Valdez Cárdenas, 50, was in his car when he was intercepted by the killers, according to Ríodoce, a weekly he founded with Ismael Bojórquez in the city of Culiacán in 2003.

At least 104 journalists have been murdered in Mexico since 2000, while 25 others have disappeared, according to the press freedom organization Article 19.

The death of Mr. Valdez, who had shared prizes from Columbia University and the Committee to Protect Journalists, raises pressure on the government of President Enrique Peña Nieto to address the killings more forcefully.