As the local cartel raced up the road to confront its enemies, the family stopped at the S.U.V.’s smoldering frame, where the remains of Mrs. Miller and her children were scarcely recognizable.

Social media began to light up as family members shared the tragic news on WhatsApp and posted videos to Twitter, pleading for help. Along the road through the mountains, cell reception is spotty at best. Within a few hours, the family was in a panic, worried about the other S.U.V.s in Mrs. Miller’s convoy. They had set out from La Mora shortly after 9 a.m. and hadn’t been heard from since.

Relatives called the United States Embassy, the federal police, offices of two state attorneys general and the Mexican military, leveraging any connections they could to mount a rescue.

By late afternoon, Julian LeBarón and his father formed a search party and took off from the community of LeBarón, about a three-and-a-half-hour drive, hoping to intercept the women from the opposite end of the road.

It is unclear what time the second ambush — on the two vehicles carrying Christina Langford Johnson, Dawna Langford and their children — occurred. The government says it happened around 11 a.m., perhaps an hour or more after Mrs. Miller was ambushed. By that time, the other mothers and children were about 11 miles ahead of her.

The road narrowed as it climbed steeply. To the left, a dense wall of mud rose into the hillside. To the right, a ravine plunged to the floor of a narrow valley before rising into a towering mountain.

The location seemed chosen for its vulnerability. An attack could not easily be defended — or escaped. As the women barreled up the road, their vehicles were easy targets.