It is at least the second time this year that a family has been caught up in a shooting involving Mexico's military, which has come under intense criticism for human rights abuses as soldiers fight brutal drug cartels.
Javier Trevino, Nuevo Leon state lieutenant governor, said soldiers apparently shot at the car when the driver failed to stop at the checkpoint Sunday on the highway connecting the northeastern city of Monterrey to Laredo, Texas.
But the boy's mother and uncle said the family had just passed a military convoy when the soldiers opened fire.
Patricia Castellanos, a 45-year-old tamale vendor, said they were returning to Monterrey after visiting her sister in the town of Salinas Victoria. Castellanos said she and her husband were traveling with their teenage son, their 24-year-old daughter, their son-in-law and their two grandchildren, ages 8 and 9.
The 15-year-old, Alejandro Leon, was shot in the head and back and died at the scene. Castellanos' husband, bricklayer Vicente Leon, was struck in the back and died at the hospital.
"I don't know how to explain the pain I'm feeling," said Castellanos, her eyes welling as she sat with her brother in her small home in a working-class neighborhood on the outskirts of Monterrey. "Because of a mistake, they suddenly ended my son's life and my husband's life."
She had gauze wrapped over her shoulder where a bullet grazed her.
A small photograph of Alejandro Leon wearing a cap and gown from his junior high school graduation sat on a bookcase.
His uncle, Luis Castellanos, said the boy was a good student and his parents worked hard to keep him in school.
"Everyone asks for justice. But who ever gets justice?" he said.
The Defense Department promised an investigation and expressed its "deep condolences to the family" in a statement.
Patricia Castellanos said a military commander had visited her in the hospital to apologize, and the state government offered to help pay medical and funeral expenses.
"An apology doesn't remedy anything," Luis Castellanos said. "The military had gained our respect, but now when you see them, all you feel is fear."
Patricia Castellanos' daughter, Iliana Leon, was cut in the face by flying glass and was awaiting plastic surgery at a hospital. The two young children, Iliana's sons, were being treated for cuts and shock.
This latest shooting comes as the military faces a controversy involving the April death of two brothers, ages 5 and 9, on a highway in Tamaulipas, a state bordering Nuevo Leon.
The National Human Rights Commission accused soldiers of shooting the children and altering the crime scene to try to blame the deaths on drug cartel gunmen.
The army denies the allegations and says the boys were killed in the crossfire of a shootout between soldiers and gunmen.
The scandal has renewed demands from activists that civilian authorities, not the army, investigate human rights cases involving the military.
More recently, soldiers killed a U.S. citizen Aug. 22 outside the Pacific coast resort city of Acapulco.
In a statement to police, an army lieutenant claimed that Joseph Proctor, who had lived Mexico for several years, shot first at the military convoy on a highway between Acapulco and Zihuatanejo.
The Defense Department said it was investigating the officer's claim, which Proctor's father, William Proctor, said he found hard to believe.
The administration of President Barack Obama is withholding $26 million in aid to Mexico, recommending that the government give more power to its human rights commission and crack down on abusive soldiers.
In a report released last week, the State Department said the Mexican government has met human rights requirements to receive $36 million in previously withheld funds that are part of a $1.4 billion Merida Initiative.
But the U.S. said it would withhold 15 percent of newly authorized funds until Mexico meets several requirements: enhancing the authority of the National Human Rights Commission, limiting authority of military courts in cases involving abuse of civilians, and improving communication with human rights organizations.
The Mexican government said it is working to improve human rights but noted in a statement that cooperation between the two countries "is based on shared responsibility, mutual trust and respect for the jurisdiction of each country, not on unilateral plans for evaluating and conditions unacceptable to the government of Mexico."
(This version CORRECTS spelling of official's name to Trevino, not Trevina, and his position to lieutenant governor, not government secretary)