Cárdenas was attacked in the state of Sinaloa, where he lived and worked. The area in northwestern Mexico is known as the country’s drug capital.
Mexican President Enrique Peña Nieto condemned the slaying in a tweet, and expressed condolences to Cárdenas’ family.
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. He founded RioDoce in 2003, which has won several awards. He also won an International Press Freedom award in 2011.
He compiled much of his reporting into several books, the most recent of which, Narcoperiodismo, was published late last year. It tells the stories of Mexican journalists who have been victims of crimes.
Several other journalists in Mexico have been killed in recent months. Three were slain in March.
“In Mexico a ‘war’ is raging against journalists,” Erika Guevara-Rosas, Americas director at Amnesty International, said in March. “The country has turned into a no-go zone for anyone brave enough to talk about issues including the increasing power of organized crime and the collusion of these groups with the authorities.”