The Arizona Daily Star reported that 18 vehicles carrying about 100 people entered the U.S. via the port of entry in Douglas, Ariz.
Bryce Langford, whose mother, Dawna Ray Langford, was one of the women killed Monday, told the Daily Star he was on his way to visit his brother at a hospital in Tucson. He said most of the families are traveling to Phoenix, and others are heading to Tucson. They are not sure where they will settle down in the long term.
The families had lived in the hamlets of La Mora and Colonia LeBaron, two communities in Mexico's Sonora state that were left grieving after the massacre that left three women and six children dead. Other residents of the hamlets planned to depart in the coming days.
The Americans were riding in a convoy of SUVs when attackers opened fire on a dirt road from La Mora leading to Colonia LeBaron. Gunmen from the Juarez drug cartel had apparently set up the ambush as part of a turf war with the Sinaloa cartel, and the U.S. families drove into it.
Mexican officials said the attackers may have mistaken the group's large SUVs for those of a rival gang.
About 300 people live in the small community of La Mora, whose residents consider themselves Mormon but are not affiliated with The Church of Jesus Christ of Latter-Day Saints.
The spread-out community in Mexico traces its origins to the end of polygamy more than a century ago by The Church of Jesus Christ of Latter-day Saints, forcing Mormon families in the U.S. with multiple wives to establish offshoots elsewhere.
Much of the area is generally without law enforcement most of the time, and residents have taken to providing their own security since the 2009 killing of an anti-crime activist.
It took Mexican soldiers more than 8 hours to respond to Monday's attacks -- the nearest units were about 100 miles away at the time. Five surviving children hid in the mountains with bullet wounds.
Langford, who was raised in La Mora but now lives in North Dakota, said the community has learned more about cartel hitmen in the area in recent months, and people had been considering moving. Monday's tragedy was the final straw.
"The assets that they've acquired down there are tremendous," he said. "And to have to up and leave from one day to the next and leave all that behind, there's definitely a lot of sad people here."
The victims were buried this week. Titus and Tiana Miller, 8-month-old twins, were laid to rest Friday in Colonia LeBaron with their mother, Rhonita “Nita” Miller, along with their siblings, 12-year-old Howard Jr. and 10-year-old Kristal. Langford, 43, and her two sons, 11-year-old Trevor and 2-year-old Rogan, were buried Thursday in La Mora. Christina Langford Johnson was buried Saturday morning.
Fox News' Paulina Dedaj and The Associated Press contributed to this report.