Ciudad Juarez, Mexico (CNN) -- Two U.S. citizens -- a mother and her son --- have been killed in the violence-plagued Ciudad Juarez, Mexico, authorities said.
Rosa Williams, 35, and her son, Pablo Noe, 19, were among four people who were killed by assailants armed with an AK-47 rifle and 9 mm pistol who opened fire on a blue 2004 Dodge Durango SUV in which they were riding, said Arturo Sandoval, a spokesman for the prosecutor's office in the Mexican state of Chihuahua.
Authorities identified them in a statement issued Monday. They identified the other two victims as Alberto Nieto Nieto, 24, and Alma Yesenia Flores, 21.
The attack happened Saturday in front of dozens of witnesses on a busy road in Ciudad Juarez, which is just across the U.S.-Mexico border from El Paso, Texas.
Investigators have not determined a motive for the shooting.
Juarez is considered a flash point in Mexico's nearly five-year-long war on organized crime.
The U.S. Consulate General in Ciudad Juarez issued a warning this year for American citizens in the city and other parts of the state of Chihuahua.
The message cautions that recent successes against drug cartel figures could put Americans in Mexico at risk.