MEXICO CITY  Gunmen ambushed a peasant union leader and his family in northern Mexico, killing him and 14 others in an attack that bore the hallmarks of a hit by drug cartels but one that his organization said might have been related to his union work.

The victims, who included four children, were leaving a ranch in the northern state of Sonora on Friday afternoon when the gunmen opened fire with AK-47s on a convoy of three trucks, according to one of the three survivors, the state police said.

The union leader, Margarito Montes Parra, 56, had built up a powerful organization spanning several states to demand land rights and press for government support of peasants. The group, the Worker, Peasant and Popular General Union, is based in the southern state of Oaxaca, and a leader there said Mr. Montes’s killing was linked to his work.

“There is evidence to believe that he was executed by someone who never agreed with our struggle,” Karina Barón, who runs the union’s Oaxaca office, told local newspapers.