MONTERREY, Mexico (AP) — Mexican authorities continued searching Monday for the gunmen responsible for an attack on a sparsely traveled stretch of highway near the Texas border that left a 13-year-old U.S. citizen dead and four relatives wounded.

On Saturday night, a family traveling in two vehicles was attacked on a two-lane highway paralleling the U.S.-Mexico border in the township of Ciudad Mier.

One SUV of attackers passed the family and then cut them off causing them to collide and come to a halt. Gunmen then opened fire, according to a statement from the state of Tamaulipas security coordinating group. All of the wounded came from one of the family's vehicles, both of which had Oklahoma license plates. The gunmen escaped in another vehicle.

A 10-year-old relative was among those wounded. They were all taken to a hospital in Monterrey after initially being moved to a nearer small town hospital.

On Monday, Luis Enrique Orozco, a deputy in the Nuevo Leon state prosecutor's office, said the boy was in critical condition along with a 48-year-old man. A 42-year-old woman was in stable condition, he said. The relationship between the adults and the children was unclear, but Orozco said the two wounded adults were siblings.

The family was returning to the U.S. after spending the holidays in the central Mexico state of San Luis Potosi. What remained unclear was why the family was on such a dangerous stretch of highway after dark.

The area where the attack occurred is contested by drug cartels.

For years Ciudad Mier was the uppermost edge of the Gulf cartel’s control and Nueva Ciudad Guerrero was the limit for the Nuevo Laredo-based Zetas’. Between them sits uninhabited scrub land.

In 2010, after the Zetas split from the Gulf cartel and established themselves as an organized criminal power through prominent displays of graphic violence, Mier became a battleground for the two cartels and most of its residents abandoned the quaint colonial town.

More recently, however, the Zetas’ splinter group known as the Northeast cartel has been as far downriver as Mier, Miguel Aleman and Camargo, well into what was traditionally the Gulf cartel’s territory.

Photographs from Saturday night’s crime scene showed the Northeast cartel’s Spanish initials — “CDN” — scrawled on the back window of one of the vehicles.

The latest U.S. State Department travel advisory in mid-December listed Tamaulipas state as “do not travel” due to the prevalence of crime and kidnapping.

“Heavily armed members of criminal groups often patrol areas of the state in marked and unmarked vehicles and operate with impunity particularly along the border region from Reynosa northwest to Nuevo Laredo,” the advisory said. That was where the family was driving.

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AP writer Christopher Sherman in Mexico City contributed to this report.