A total of 20 bodies have been found in 15 clandestine graves in a town near Ciudad Juarez, a border city in the northern Mexican state of Chihuahua, officials said.

Several of the victims, who were apparently murdered more than two years agon, were U.S. citizens, "but they have not yet been completely identified," prosecutor Jorge Gonzalez Nicolas said.

The bodies were found in La Colorada, a town in the Juarez Valley, Gonzalez said.

Eleven bodies were initially discovered at the site last Saturday, but investigators later found nine more sets of human remains.

Information about the clandestine graves was obtained from the U.S. Consulate in Ciudad Juarez, located across the Rio Grande from El Paso, Texas, and U.S. investigators, who were tipped off to the graves by informants and individuals arrested on drug charges, Gonzalez said.

The bodies of four people, meanwhile, were found Monday on the highway that leads to Creel, a town in Chihuahua, state officials said.

Gunmen in Chihuahua city, the state capital, killed five people riding in an automobile early Monday.

Chihuahua has been one of the states most affected by the wave of drug-related violence in Mexico.

More than 11,000 people have been murdered in Ciudad Juarez, located across the border from El Paso, Texas, since 2008.

About 50,000 people have died in drug-related violence in Mexico in the past six years, the majority of them in shootouts involving members of rival drug cartels. EFE