The groups were fighting over territory in Nezahualcoyotl for local drug sales, said Alfredo Castillo, attorney general of Mexico state, which borders the Mexican capital. He said the same dispute caused a shootout that killed eight in January in the same suburban area.
It was the first time authorities have linked a major cartel with the recent rise in attacks outside Mexico City, which has largely been spared from country's drug violence.
La Familia controls drug trafficking, extortion and other crime in the Pacific coast state of Michoacan.
Castillo said La Familia was trying to expand into several suburbs of Mexico City, engaging in drug dealing, extortion and car theft.
"Drug traffickers have been trying to penetrate Mexico state to reach the capital," Castillo said.
His comments comes despite the federal government's claim that La Familia is in retreat after arrest of several top members and the death of its leader Nazario Moreno in a December gunbattle with police.
For months, letters dropped on the streets of Michoacan towns claimed that La Familia was seeking a truce with the government. Banners appeared around Michoacan last month claiming La Familia had dissolved itself, although some state lawmakers dismissed the messages as a ploy to distract attention from the cartel.
La Familia burst onto the national scene in 2006 when it declared its independence from the Gulf cartel by rolling five severed heads into a disco in the mountain town of Uruapan.
Shortly afterward, President Felipe Calderon deployed thousands of troops to Michoacan, the start of a campaign to destroy cartels in their strongholds throughout Mexico. Drug gang violence has since soared to unprecedented levels, claiming nearly 35,000 lives nationwide.
La Familia's apparent move into Mexico City outskirts comes amid the splintering of another cartel - the Beltran Leyva gang - that has largely controlled the area stretching from the Pacific coast resort of Acapulco to just south of the Mexican capital.
The Beltran Leyva gang has been in disarray since the shooting death of its leader Arturo Beltran Levya in a December 2009 battle with marines.