The U.S. State Department is ordering the family members of officials in six cities on the Mexico-U.S. border to return home.

The order comes after an American consulate worker and her husband were gunned down in their car in the border city of Juarez, Mexico on Sunday. Their one-year old child was found crying, but unharmed, in the back seat.

Meanwhile, in a separate shooting, the husband of another U.S. official was also killed.

The killings took place in the midst of an escalating drug war in Mexico.

President Barack Obama expressed outrage over the killings, the White House said in a statement.

"He extends his condolences to the families and condemns these attacks on consular and diplomatic personnel serving at our foreign missions," the statement said. "In concert with Mexican authorities, we will work tirelessly to bring their killers to justice."

Secretary of State Hillary Clinton said "these appalling assaults on members of our own State Department family are, sadly, part of a growing tragedy besetting many communities in Mexico."

"They underscore the imperative of our continued commitment to work closely with the Government of President Calderon to cripple the influence of trafficking organizations at work in Mexico," she added. "This is a responsibility we must shoulder together."

Meanwhile, the State Department has also issued a travel warning for Americans traveling to Mexico.

Thousands of college students are expected to travel there for spring break in coming weeks.