Mexico’s drug wars crossed the border yesterday as American officials removed their families from a string of northern consulates and the FBI joined the hunt for the killers of the first US Government employees to die in a battle between crime cartels.

A seven-month-old baby, found crying in the back of a bullet-riddled car, was among the few survivors of two hit-and-run shootings that took place minutes apart in the border city of Ciudad Juárez at the weekend. She was being cared for by relatives while funeral arrangements were being made for her parents, who were found in the front of the car with multiple gunshot wounds to the head and neck.

President Obama has expressed his outrage at the killings and said that the US would work tirelessly to bring those responsible to justice. That will not be easy in Ciudad Juárez, the world’s most dangerous city. More than 2,500 people died last year in a war between the Juarez and Sinaloa drug cartels for control of the Juárez-El Paso border crossing in west Texas and the lucrative distribution routes that fan out from it throughout the US.

There was speculation that the recent extradition to America of several Mexican drug barons may have been behind the murders of Lesley Enriquez, a worker at the US consulate in Ciudad Juárez, and her husband, Arthur Redelfs, as they drove away from a children’s birthday party.

Witnesses said that they saw a group of armed men in a vehicle begin following them at 2.42pm on Saturday. The couple attempted to escape, apparently intending to cross the border into the US, but were fired on when they tried to turn a corner and crashed when they swerved into oncoming traffic.

Minutes later the Mexican husband of another consular worker was shot dead while driving home from the same party.

Vladimir Tuexi, the spokesman for the state prosecutor, said that Ms Enriquez’s baby had been cared for by the Mexican social services before being handed to relatives. Robert Cason, Mr Redelfs’ stepfather, refused to comment on the welfare of the child. “I don’t want to give any information to the psychotics out there,” he said.

The consulate in Juárez has been temporarily closed after the killings and employees of five US consulates in the northern states of Mexico have been told that they can send their families home. The US has 485 locally engaged consulate staff in Mexico and its Consulate General in Ciudad Juárez is the largest operation in the world, handling 800 visa cases a day.

A US spokesman in Monterey, northeast Mexico, said that the evacuation was voluntary but added: “If any relative of an American official in any of these consulates wants to leave, the American Government will pay for them to do so.”

Drug gangs have killed thousands of people in Mexico since the beginning of President Calderón’s term in 2006. The violence, which has turned neighbourhoods into war zones, has come despite $1.3 billion (£860 million) in law enforcement aid from the US and repeated pledges by Mr Calderón to get tough with the cartels.

Although politically significant, the consulate workers’ deaths were the latest in a wave of murders that left at least 17 dead at the weekend in the Pacific resort of Acapulco alone.

The Chihuahua State Government blamed the Ciudad Juárez killings on a gang known as the Aztecas, although the motive remained a mystery.

Although American citizens are often the victims of violent crime in Mexico — with an average of one dying every week — this is the first time that a consulate employee has been killed. Also at the weekend in Juárez, armed men opened fire on a funeral, killing six people including a two-month-old baby and a 14-year-old girl.