(CNN) A 13-year-old American boy was killed in Mexico while traveling with his family back to the United States, the boy's father and authorities said.

Oscar Lopez was shot in both legs and died in an attack Saturday night in Tamaulipas state, near the US border, said the teen's father, also named Oscar Lopez.

The father said three other relatives were injured in the ambush attack: his wife, 41-year-old Juanita Lopez; their other son, 11-year-old Abio Lopez; and his brother-in-law, 48-year-old Raphael Castillo.

The elder Oscar Lopez said the group was visiting relatives in the Monterrey area for the holidays, though he stayed back in the US.

On Saturday night, family members in two vehicles -- both with Oklahoma license plates -- were driving on the Reynosa-Nuevo Laredo Mexican highway in Ciudad Mier, according to a statement from Tamaulipas state authorities.

The family was confronted by unknown assailants. Tamaulipas officials said the family's van did not stop, so the assailants crashed into the van to force it to a halt.

"Armed individuals detained them and shot them," Tamaulipas authorities said in their statement.

Mexican officials said three people injured in the attack were in stable condition at a Nuevo Laredo hospital.

The elder Oscar Lopez said his wife was shot in the stomach, and both his younger son and his brother-in-law were injured in the legs.

Investigators found three vehicles impacted by the crash -- two with Oklahoma plates, and one with a Tamaulipas plate that authorities believe was used by the attackers.

The attackers' whereabouts were not immediately known. The slain boy's father said he has no idea why his loved ones were targeted.

A US State Department official confirmed one US citizen was killed in the attack.

"We offer our sincerest condolences to the family on their loss, and we are providing them all appropriate consular assistance," the State Department official said.

"We are closely monitoring local authorities' investigation into this brutal attack," the official added. "Out of respect for the family's privacy, we have no further comment at this time."