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 .
A mother and her son were killed in an ambush . They were among 4 killed in the attack in Ciudad Juarez . Authorities identified the victims Monday . Investigators have not determined a motive .