The women and children massacred Monday in northern Mexico were members of a decades-old settlement founded as part of an offshoot of the Church of Jesus Christ of Latter-day Saints.

La Mora, a small community in Sonora state about 70 miles south of the border of Douglas, Ariz., where families farm pecans and raise cattle, was established by some of the original pioneers – later excommunicated – sent by the Mormon church in Utah to colonize northern Mexico in the late 1800s.

Matthew Bowman, an associate professor of history and religion at Claremont Graduate University in California, told the Associated Press communities like La Mora started forming in the 1870s, when the U.S. government started prosecuting polygamists.

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The Church of Jesus Christ of Latter-day Saints responded by settling colonies outside the U.S. in Mexico and Canada, instead.

The church abandoned polygamy in 1890, though some members continued to practice it. In the 1920s, church leaders mounted an aggressive campaign against those members, excommunicating some and applying church discipline, Bowman said.

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That sent some fundamentalists into northern Arizona and others further south, to Mexico.

The most notorious family from La Mora community are descendants of Alma LeBaron, now settled in the sprawling Colonia LeBaron.

Alma LeBaron moved his family from Utah to Chihuahua in the early 20th century and, despite his efforts, he died excommunicated. LeBaron was the grandson of Benjamin F. Johnson, who had worked closely with the founder of the Church of Latter Day Saints, Joseph Smith. After Alma LeBaron's death, his sons established the fundamentalist Church of the Firstborn of the Fullness of Times, Bowman said.

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The nine women and children killed Monday by the cartel gunmen were part of the remote community.

Among those killed were Rhonita Maria Miller, with ties to the LeBaron family, and her four children. Her burned-out bullet-ridden SUV was reportedly found outside the town of Bavispe, where the settlement is located.

Dawna Ray Langford, 43, and Christina Marie Langford, 31, were also among those killed.

The Langford family descends from the pioneers who established "Colonia Oaxaca," which is now called "Rancho Oaxaca," a famous Mormon colony, and the families are part of remnants from that.

The majority of the dual U.S. citizens who live there go back and forth between Utah, other parts of the United States, and their community in La Mora. The women were traveling back to Utah when they were confronted by the cartels.

Lafe Langford Jr., a relative, spoke to Fox News.

"It's just huge. It's just absolutely unimaginable," Langford said. "This is the absolute worst nightmare for our entire existence in Mexico ... and we never thought it was possible."

He added: "There's thousands of people affected by this. Just American citizens in Mexico, there's thousands affected by this."

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Langford said their families have never had "even the slightest interactions with the drug cartel in the last 70 years" and that they are loved by their community, serving the Mexican people.

"What we want right now is answers. Who and why. It was an attack on innocents, women and children."

But 10 years ago, Benjamin LeBaron, the founder of crime-fighting group SOS Chihuahua, was killed in the neighboring Chihuahua after he led protests over the kidnapping of their 16-year-old brother. The Mormons refused to pay the ransom and the young LeBaron was ultimately released.

The community of La Mora has about 350 people who live there full time. The rest, about 1,000 members, live throughout the United States.

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The La Mora community is composed mostly of non-practicing members, as many come from families who were excommunicated for polygamy. They are located outside the church perimeter yet they still practice Sunday School in their homes.

"What sets us apart from the fundamentalist groups is that we don't claim priesthood authority and prophets and apostles outside of the Mormon church," Langford said. "We mainly believe the Mormon church is out of order and that's why we're excommunicated."

Sen. Mitt Romney, R-Utah, whose own father was born in a Mormon settlement in Mexico, extended his condolences for the victims and called for the U.S. to get involved.

“Our prayers are with their families who have suffered such an unspeakable tragedy. The U.S. must work with Mexican officials to hold accountable those responsible for this senseless violence,” he tweeted.

A spokesman from the Church of Jesus Christ of Latter-day Saints confirmed to Fox News that the La Mora community is an offshoot of their church.

Fox News' Danielle Wallace, Adam Shaw, and The Associated Press contributed to this report.