Press freedom groups are demanding that Mexico investigate the shooting death of a freelance journalist just after he reportedly witnessed a confrontation at a highway checkpoint.

The New York-based Committee to Protect Journalists says reporter Adrian Silva was found dead in his car Wednesday.

Silva had been covering a story on gasoline thefts from Mexico's state-owned oil company. He worked for several publications in the central state of Puebla.

The committee said Thursday that Silva was returning from covering an army investigation into the thefts when he told a fellow journalist he had just witnessed a confrontation between gunmen and soldiers at a highway checkpoint.

The killing apparently occurred moments later.

In July, a special prosecutor said 67 journalists have been killed and 14 have disappeared in Mexico since 2006.