CIUDAD JUAREZ, Mexico (AFP) – US agents joined a probe into the brazen weekend killings at an American consulate in Mexico's bloodiest city, highlighting rampant violence that prompted expressions of concern from both governments.

Officials from the Federal Bureau of Investigation (FBI) arrived in violence-plagued Ciudad Juarez to probe the fatal shootings of two American and a Mexican with ties to the US consulate -- the first such attack in recent memory on American diplomatic personnel.

Mexican authorities blamed the drive-by murders of the American employee of the US consulate, her husband and the husband of a Mexican consular employee on "the Aztecas," a gang linked to the powerful Juarez drug cartel.

But investigators said it was still unclear why the victims were singled out by hit teams who ambushed the two family groups just minutes apart Saturday after they left a birthday party in Ciudad Juarez.

"It could be a mistaken identity, it could be that they were targeted; we don't know at this point," said special agent Andrea Simmons of the FBI's El Paso, Texas, office just across the border from Ciudad Juarez.

She said seven or eight FBI agents had joined the investigation, along with agents from the US Drug Enforcement Administration and the Bureau of Alcohol, Tobacco and Firearms.

Meanwhile the bodies of the two Americans slain in the attacks were taken across the border to El Paso for post-mortem examinations.

Police on Monday also located the charred van in a residential section of Juarez which they believed was used in the hit job. The van, which had been reported stolen by its owners, was found about two kilometers (1.2 miles) from where the Saturday's shooting took place on the outskirts of the city.

According to neighbors, the driver of the van, along with two hooded men who followed in separate vehicles, doused the car in gasoline and set it ablaze.

The shocking attacks, which occurred in broad daylight, came amid escalating violence in Juarez, where a raging drug war has claimed thousands of lives in the past few years.

Violence here ratcheted up almost from the start of President Felipe Calderon's administration in December 2006, after his decision to deploy the military to crack down on drug traffickers who had operated for years with near impunity.

Calderon, who was due Tuesday to travel to the troubled border city for the third time in two months, reaffirmed Mexico's "commitments to solve these crimes."

"We will, as the secretary and president pledged, work tirelessly with Mexican authorities to bring the killers... to justice," State Department Spokesman Philip Crowley said in Washington.

The State Department also updated its existing travel warnings, warning Americans against travel to northern Mexico and strongly cautioning American students against spending their spring break holidays in Mexico.

US officials moved to close the facility pending the outcome of a security review.

The shooting victims were identified as Lesley Enriquez, an American working at the consulate; her American husband, Arthur Redelfs; and Jorge Alberto Sarcido, the Mexican husband of another consular employee.

Enriquez and her husband were killed in a hail of bullets as they were driving back to the US side of the border with their one-year-old daughter in the back seat, officials said. The baby survived unharmed.

In a separate attack, gunmen opened fire on Sarcido's car, killing him and wounding his two children, ages four and seven. His wife, a Mexican employee of the consulate, was following in a second car and escaped injury, a US official said.

President Barack Obama said Sunday he was "deeply saddened and outraged" by the killings, which marked an ominous turn in an already bloody war waged by the cartels on rivals and authorities.

More than 2,600 people were murdered in Ciudad Juarez in 2009 in drug-related violence as the cartels battle for control over the lucrative smuggling routes into the United States.

Drug-related violence claimed more than 100 lives across Mexico just last weekend, police data showed.

Among the hardest hit areas were Guerrero state, home to the major tourist destination of Acapulco, where the notorious "La Familia" drug cartel is active. There were 45 weekend murders in Guerrero, police said.