The video could not independently be verified.
"It was a massacre", Julian LeBaron, an activist who has denounced criminal groups in the area, told Mexican radio network Formula.
The girl who went looking for Devin, Mckenzie, was found after a two-hour search by relatives and soldiers after losing her way in the dark, Miller said.
"We don't know why, though they had received indirect threats".
A relative of the dead who did not want his name used for fear of retaliation said in an interview that when they finally made it to the scene where the ambush started - about 11 miles (18 kilometers) from where the two other mothers were killed - they found a burned-out Chevy Tahoe.
"We've been here for more than 50 years". Whoever did this was aware.
A relative of the victims told the Associated Press that members of the community had been travelling in a convoy of three vehicles when the attacks occurred.
A quick-thinking 13-year-old boy hid his bleeding siblings in the bushes and a mother saved her 7-month-old baby before she was killed, according to a relative of those murdered in Mexico during the horrific cartel massacre of USA citizens Monday.
Here, President Andres Manuel Lopez Obrador looks on as Security Minister Alfonso Durazo speaks at a news conference about the attack Tuesday morning.
There is a GoFundMe to help support families of the victims.
During a live news conference alongside one of his senior security officials, Mexican President Andres Manuel Lopez Obrador said Tuesday that he did not believe his country would "need foreign intervention to deal with such cases", but that he was prepared to allow Mexican authorities to work with their US counterparts, provided Mexico's independence was respected.
Lopez Obrador's comments came shortly after Trump tweeted that it was time for Mexico to wage war on drug cartels and "wipe them off the face of the earth" with "help" from the US. Lopez Obrador has emphasized anti-poverty programs as a means to reducing cartel membership, but many, including security expert Edgardo Buscaglia of Columbia University, point out that many states poorer than Mexico have lower crime rates, so poverty alone can not be to blame for the unfolding turmoil in Mexico.
The government deployed the army to fight drug trafficking in 2006, but experts blame the so-called "drug war" for the spiraling violence between fragmented cartels and the military, which has led to more than 250,000 murders. "This is a whole new level", said Taylor Langford, a relative of the dead who splits his time between the Mexican community and his home in the Salt Lake City suburb of Herriman, Utah.
"The worst that we could see is war", he said.
Mormons of Germanic origin left the USA and established a settlement in northern Mexico.
"We still suffer from insecurity and violence", he said.
"One of the issues with these Mormon communities down in Mexico is that they are all not affiliated with one Mormon church", said Benjamin Park, assistant professor of history at Sam Houston State University.