US FBI agents have joined a Mexican investigation into attacks on US consular staff and their families that left three dead in the border city of Ciudad Juarez.
Mexican authorities blamed the drive-by murders of an 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 they were singled out by hit teams who ambushed the two family groups just minutes apart on Saturday, after they left a birthday party in Ciudad Juarez.
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"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, a spokesperson for the FBI's El Paso, Texas, office just across the border from Ciudad Juarez.
She said seven or eight agents of the Federal Bureau of Investigation had joined the investigation along with agents from the US Drug Enforcement Agency and the Bureau of Alcohol, Tobacco and Firearms.
Killers' burned-out van found On Monday, police located the charred van in a residential section of Ciudad 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 from where the shooting took place.
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.
In Washington, State Department spokesman Philip Crowley said the Ciudad Juarez consulate, one of the largest such US facilities in the world, would remain closed Tuesday as it undergoes a security review.
The city's mayor told reporters that security had recently been beefed up at the consulate following a bomb threat.
"We will, as the secretary and president pledged, work tirelessly with Mexican authorities to bring the killers... to justice," Crowley said.
President Felipe Calderon, who is expected to travel to the troubled northern city on Tuesday, for the third time in two months, reaffirmed Mexico's "commitments to solve these crimes".
Baby orphaned in shooting
The victims were identified as Lesley Enriquez, an American working at the Juarez 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.
US President Barack Obama said on Sunday he was "deeply saddened and outraged" by the killings, which marked an ominous turn in an already bloody war waged by drug cartels on rivals and the 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.
Deadly drug gang battle
More than 15,000 have died across Mexico over the last three years in an escalating and often shockingly brutal drug war, according to government figures.
"We don't have any indication that these individuals (who were slain) were specifically targeted," said a US administration official in Washington.
Although the motive for the latest killings was unknown, several prominent drug kingpins have been recently extradited by Mexico to the United States to stand trial.
Jesus Vicente Zambada-Niebla, son of Sinaloa Cartel chief Ismael "el Mayo" Zambada-Garcia, appeared last month in a Chicago court on drug trafficking charges.
Miguel Caro Quintero, a brother of another notorious Mexican drug baron, Rafael Caro Quintero, was sentenced in a Colorado court to 17 years in jail.
The US State Department authorized US staff in six consulates along the US-Mexican border to send their dependants home for safety.
It also warned US citizens to "delay unnecessary travel" to parts of Mexico that have seen an increase in violence.
