Gunmen killed a U.S. consulate employee and her husband as they drove in Ciudad Juarez with their baby in the back seat, minutes after the husband of another consular employee was shot to death and his two children wounded, officials said Sunday. Security forces suspected a drug gang hit, but offered no motive.

President Barack Obama expressed outrage over the killings, and Mexican President Felipe Calderon promised a swift investigation.

The gunmen are suspected of belonging to a gang of hit men tied to the Juarez drug cartel. But police gave no information on a possible motive. U.S. State Department spokesman Fred Lash said the three slain people had attended the same social event before the attacks Saturday.