The three victims died in drive-by shootings in two separate attacks
Suspected drug cartel hit squads gunned down three people connected to the US consulate in the Mexican border city of Ciudad Juarez, a US official says.
They include a consulate employee and her husband, both American, and the Mexican husband of another employee.
President Barack Obama said he was "outraged" by the drive-by shootings.
It is not yet clear if they were specifically targeted, but the US has authorised employees at some consulates to send family members out of the area.
The six consulates are in the border cities of Tijuana, Nogales, Ciudad Juarez, Nuevo Laredo, Monterrey and Matamoros.
Mexico is battling a drug war that has killed some 18,000 people since 2006.
Ciudad Juarez is a major flashpoint in the battle between Mexican drug gangs over trafficking routes to the US. More than 2,600 people were murdered there in drug-related violence last year alone.
The three victims were killed in drive-by shootings in two separate attacks on Saturday afternoon, a US official said on condition of anonymity, citing privacy concerns.
"The president is deeply saddened and outraged by the news," White House National Security Council spokesman Mike Hammer said in a statement.
"In concert with Mexican authorities, we will work tirelessly to bring their killers to justice," he added.
A relative has identified the American couple as 35-year-old Lesley Enriquez and her husband Arthur Redelf, 34, of El Paso, Texas. Their one-year-old baby was found unharmed in the back seat.
In the second attack, gunmen opened fire on the car belonging to the husband of a Mexican consulate employee. They killed the man and wounded his two children, the official said.
"Suspected drug cartel hit teams fired on locally employed staff... in their privately owned vehicles," the US official said.
"Both families had attended the same social event earlier in the afternoon off-post away from the consulate... It has not been determined if the victims were specifically targeted," the official added.
Shortly after the killings were disclosed by the White House, the state department issued a travel warning for Mexico.
It urged US citizens to delay unnecessary travel to parts of the Mexican states of Durango, Coahuila and Chihuahua due to the recent violent attacks.
On Saturday, 13 people were killed in an outbreak of drug-related violence in the southern Mexican beach resort of Acapulco, officials said.
Acapulco is one of Mexico's biggest tourist resorts, but in recent years it has been the scene of bloody turf wars between rival drug cartels.