Ciudad Victoria, Tamaulipas, Mexico — An American family returning to the U.S. after a holiday visit to Mexico came under attack just south of Texas on Saturday night, with armed gunmen killing a 13-year-old U.S. citizen and wounding four relatives. A U.S. State Department official told CBS News two of the wounded are American citizens.

The official didn't identify any of the victims. The State Department was monitoring the investigation into the attack, the official said.

The family was traveling in two vehicles. They were attacked on a two-lane highway paralleling the U.S.-Mexico border in the township of Ciudad Mier.

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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. On Sunday, authorities listed the wounded as in stable condition.

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 highway on which they were shot is considered high risk. It runs through an area that's disputed by criminal groups, including the Gulf Cartel and Zetas. The road connects the city of Mier with Nueva Ciudad Guerrero, on the banks of the Rio Grande across the U.S.-Mexico border from Falcon Heights, Texas.

El Diario reported that the attackers were drug traffickers with the Cártel del Noreste (the Northeast Cartel), a Zetas splinter group. Photographs from Saturday night's crime scene showed the cartel's initials — "CDN" — scrawled on the back window of one of the vehicles.

In November, an ambush in northern Mexico left nine American women and children dead. Prosecutors said last week a total of seven suspects have now been detained in connection with that attack.

Tamaulipas has been a major conduit for drug shipments and has also been the scene of some of the worst massacres and fiercest fighting in Mexico's drug war. That war left 28,689 people dead in Mexico in 2017, according to government statistics — the highest number ever recorded, and likely was even more deadly.

But as CBS News' Haley Ott reported, there are at least 36,000 more people in the country who have simply vanished amid the bloodshed. Known as desaparecidos, or "the disappeared," some are abducted, others are caught in the crossfires of the cartel-related violence that permeates Mexican society.

Many are presumed dead, but without bodies, their families are left without answers, and sometimes with the grim burden of trying to find their missing loved ones' remains on their own.