Devin Langford, 13, was in the car with his family in the Mexican border state of Sonora on Nov. 4 when men who appeared to be wearing bulletproof vests started shooting at them with long guns, the boy told ABC News in his first interview since the attack.
The massacre killed three women and six children, including Devin's mother, Dawna Langford, and his younger brothers Trevor, 11, and Rogan, 2. All nine were U.S. citizens who were part of a Mormon offshoot group living in La Mora, Mexico.
Devin said his mother's final words to her children before she was killed were "get down right now."
"She was trying to pray the Lord, and she was trying to start the car to get out of there," he said. But the car wouldn't start, he said, adding that he believed a bullet hit the engine.
"They just started hitting the car first with a bunch of bullets, just started shooting rapidly at us," the teen said.
The gunmen — who Mexican authorities say were drug cartel members who mistook the victims for members of a rival gang — then drove off, Devin said. Devin had no injuries, but his siblings did. Those who survived included a baby brother with a bullet wound to the chest and a sister who was shot in the foot.
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Devin said he and the others who were able to walk carried their injured siblings as far as they could.
"We walked a little while till we couldn't carry them no more," he said.
Fearing the gunmen would return, Devin hid the wounded children behind a bush and then started walking for help.
"Every one of them were bleeding really bad, so I was trying to get in a rush to get there," he said.
Devin ended up walking for several hours, traveling 14 miles before he found help. The whole time, he said, he was terrified, wondering if "there wasn't anybody else out there trying to shoot me or follow me."
Devin's father, David Langford, credited the boy with saving his surviving brothers and sisters.
"To be honest with you, my boy's a hero," David Langford said. "Everyone one of my children that survived are living miracles," he added.
"She was a nice person and a brave woman that tried to save her kids."
But Devin said he wanted the public to know how heroic his mother had been, too. Pausing to hold back tears, he said, "She was a nice person and a brave woman that tried to save her kids."
The attack has upended the families' lives and prompted outrage and skepticism toward Mexican authorities. Many relatives of the victims say they do not believe the shooting was a case of mistaken identity and are demanding answers.
"This was 100 percent intentional," Ryan Langford, a family member, told the "Today" show.
On Sunday, the FBI announced it was going to be "providing assistance at the invitation of the Mexican government."
"The FBI remains committed to working alongside our international partners to help bring justice to the perpetrators of this heinous act of violence," it said in a statement.
Funerals were held last week and over the weekend for the victims. In addition to Dawna Langford and two of her sons, the victims were identified as Christina Marie Langford Johnson, 29; Rhonita Miller, 30; Howard Miller, 12; Krystal Miller, 10; and infant twins, Titus and Tiana Miller.
Dozens of members of the extended family left Mexico in a caravan over the weekend, fearing it was too dangerous to stay.
"No one needs to live in a place they don't feel safe," Ryan Langford said.