At least nine U.S. citizens, including six children, were killed Monday in an apparent ambush on a highway in the Mexican border state of Sonora, according to relatives of the victims and local media reports.

The dead included 8-month-old twins, said a family member, Kendra Lee Miller. Eight children survived, some seriously wounded, including a 9-month-old who was shot in the chest and a 4-year-old shot in the back, Miller said.

Willie Jessop, who is related to one victim, told NBC News by phone from Utah that the attack was on a motorcade consisting of several families, and that survivors at the scene told him that three cars were shot at and one was set on fire.

The Mexican government said Tuesday that three SUVs were hit in the assault.

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“Everyone is in so much shock," Jessop said, adding that he has been in contact with Mexican federal officials and the FBI. "It's just unbelievable, and there's just no way to comprehend it."

Dawna Langford and her son Trevor were killed in the attack. Courtesy of Kendra Miller

Mexican government authorities said the family of three women and 14 children were on their way from Galeana, Chihuahua, to Bavispe, Sonora, and were attacked near the border between the two states.

Rhonita Miller was on her way to a shopping trip in Arizona with her family, while the others were going to Chihuahua to visit relatives, Kendra Lee Miller said.

The victims were identified by Miller as Christina Marie Langford Johnson, 29; Dawna Langford, 43; Trevor Langford, 11; and Rogan Langford, 2-and-a-half.

Also killed were Rhonita Miller, 30; Howard Miller, 12; Krystal Miller, 10; and the 8-month-old twins, Titus and Tiana Miller.

Kendra Lee Miller said a 13-year-old, Devin Langford, escaped uninjured and then walked for about 14 miles to La Mora, where the family lived, for help after hiding his wounded siblings in bushes and covering them with branches.

Mckenzie Langford, 9, was grazed in the arm with a bullet, but she also went to look for help after Devin did not come back, Miller said. Mckenzie got lost and walked for hours in the dark before she was found by search parties.

Rhonita Miller and her twins, Titus and Tiana, were killed in the attack. Courtesy of Kendra Miller

"The attacks in Northern Mexico on American citizens, including women, children, and infants, is a stark example of how these brutal organizations operate on a daily basis," Homeland Security Undersecretary David Glawe told a Senate panel on Tuesday.

"The violence and disregard for human life displayed by these criminal organizations is as barbaric and gruesome as any terrorist organization we see around the globe."

La Mora is about 75 miles south of the U.S. border. The community is descended from Mormon settlers, and some members are still practicing members of the church.

El Universal, one of Mexico's largest newspapers, quoted other relatives as saying that the family is Mormon and the attack appeared to be an organized crime ambush. There was no immediate indication of who was behind the attack.

Officials with The Church of Jesus Christ of Latter-day Saints sought to distance themselves from victims, who they consider to be from a polygamist group not affiliated with LDS.

"We are heartbroken to hear of the tragedy that has touched these families in Mexico," according to a statement from church spokesman Eric Hawkins.

"Though it is our understanding that they are not members of The Church of Jesus Christ of Latter-day Saints, our love, prayers and sympathies are with them as they mourn and remember their loved ones.

And former U.S. Ambassador to Mexico Roberta Jacobson said it's well known that breakaway Mormons have been living in the region for years.

"I mean, I think it’s important to understand in the midst of this incredible tragedy — and our hearts go out to the families — that these are dual nationals who had been living in Mexico," Jacobson told MSNBC on Tuesday.

"There has been a community of Mormons in Mexico for a long time, where people who broke off from the Mormon Church."

The ambush comes amid a wave of violence in Mexico in recent weeks, shocking even for a country used to more than a decade of intense drug-war brutality. The most notable incident was a military-style cartel assault that forced the government to release a leader of the Sinaloa Cartel in October.

Former Ambassador Jacobson said the Americans could be victims of violently reckless drug runners.

"So we don’t know the details of this particular case, but unfortunately the reaction from cartel members is often to shoot first and ask questions later about who may have been involved," Jacobson said. "And the issue of children in this, I think, is what makes it so very horrific."

In response to Monday's killings, President Donald Trump tweeted that a "wonderful family and friends from Utah got caught between two vicious drug cartels, who were shooting at each other, with the result being many great American people killed."

Trump added that the United States was "ready, willing and able" to help Mexico "in cleaning out these monsters," should Mexico want assistance.

....monsters, the United States stands ready, willing & able to get involved and do the job quickly and effectively. The great new President of Mexico has made this a big issue, but the cartels have become so large and powerful that you sometimes need an army to defeat an army! — Donald J. Trump (@realDonaldTrump) November 5, 2019

This is the time for Mexico, with the help of the United States, to wage WAR on the drug cartels and wipe them off the face of the earth. We merely await a call from your great new president! — Donald J. Trump (@realDonaldTrump) November 5, 2019

Mexican President Andrés Manuel López Obrador said that while he had not seen Trump's tweet, he was sure it was "in the spirit of cooperating... I am sure he has not been disrespectful. Every time we talk it is with that spirit of helping, which we appreciate very much."

He later, though, said a war on dugs was a bad idea.

"The worst thing is war … those who have lived war, suffered from war, know what that means… It is the opposite of politics, war is synonymous to irrationality, war is irrational," he said.

Mexico's national civil defense agency confirmed that elements of the National Guard, the army and the state police were conducting a search operation in the Bavispe municipality of Sonora, where La Mora is, on Monday night. It provided no other information.

The U.S. ambassador to Mexico, Christopher Landau, said in a tweet in Spanish that "the safety of our fellow citizens is our top priority. I am closely following the situation in the mountains between Sonora and Chihuahua."

Landau, who earlier in the day said he was on his way to Sonora, did not share details of the incident. The U.S. State Department also said it was aware of the reports but had no further comment.

"I don't know what kind of monsters dare to hurt women and children."

Claudia Pavlovich Arellano, the governor of Sonora, said on Twitter late Monday that "as a mother," she was filled with deep pain by "the cowardly acts in the mountains between Sonora and Chihuahua."

"I don't know what kind of monsters dare to hurt women and children," Pavlovich said.

Senator for Sonora, Lilly Téllez, said on Twitter “the massacre in Sonora cannot go unpunished.”

Manuel Añorve Baños, another Mexican senator, called what happened “a despicable, merciless and savage act” in a tweet, adding: “We demand justice.”