Suspected drug gang "hit teams" killed two Americans and a Mexican linked to a U.S. consulate in co-ordinated shootings that marked an ominous turn in the Mexico drug war.
U.S. President Barack Obama expressed said he was "outraged" by the latest killings in the notorious border city of Ciudad Juarez on Saturday night.
The Chihuahua state government identified the victims as U.S. consular worker Lesley Enriquez, her American husband Arthur Redelfs and Mexican national Jorge Alberto Salcido Ceniceros, husband of another consult employee in Ciudad Juarez.
A government statement said Mexican and U.S. agencies believed a gang known as "The Aztecas", which works for the Juarez Cartel, was behind the drive-by killings.
No motive was suggested, but several prominent drug kingpins have been recently extradited by Mexico to the United States to stand trial.
Jesus Vicente Zambada-Niebla, son of Sinaloa Cartel chief Ismael "el Mayo" Zambada-Garcia, appeared last month in a Chicago court on drug trafficking charges.
Miguel Caro Quintero, a brother of another notorious Mexican drug baron, Rafael Caro Quintero, was sentenced in a Colorado court to 17 years in jail.
The U.S. Congress has approved some 1.3 billion dollars for Mexico under a regional plan to fight organized crime.
The victims had all been at the same party on Saturday night, according to a U.S. official who spoke on condition of anonymity.
The slain couple was travelling with their infant daughter, who was in the back seat of the car and survived unharmed, the U.S. official said.
In a second attack, the Mexican employee was following her husband and two children in a separate car, when her husband's vehicle came under fire, killing him and wounding the two children, the official said.
The U.S. State Department authorized U.S. staff in six consulates along the U.S.-Mexican border to send their dependants home for safety.
The State Department also warned U.S. citizens to "delay unnecessary travel" to parts of Durango, Coahuila and Chihuahua states, arguing that "violence in the country has increased."
The State Department noted that some of the recent clashes resembled small-scale warfare, with drug cartels using automatic weapons and grenades.
Moreover, "drug cartels and associated criminal elements have retaliated violently against individuals who speak out against them or whom they otherwise view as a threat to their organizations," the statement warned.
Obama said that he was "deeply saddened and outraged by the news of the brutal murders."
Secretary of State Hillary Clinton offered her "deepest sympathies" to the families of those slain.
"These appalling assaults on members of our own State Department family are, sadly, part of a growing tragedy besetting many communities in Mexico," she said in a statement.
Ciudad Juarez, population 1.3 million, is a major hub for smuggling illegal drugs into the United States. It is directly across the border from El Paso, Texas.
More than 2,600 people were murdered in Ciudad Juarez in 2009 in drug-related violence.
The war between rival drug cartels to control border crossing points, as well as the government's attempt to crack down on the cartels, has left more than 15,000 dead across Mexico over the last three years, according to government figures.
This past weekend, the drug war claimed 31 more victims in several shootings that occurred in and around the famous Mexican resort of Acapulco.
About 1,000 college-age U.S. youngsters are reported to be in Acapulco this week for the traditional spring break.