The FBI has opened an investigation into the massacre of US citizens gunned down by drug cartels in northern Mexico — as the death toll has climbed to nine, including three women, 8-month-old twins and four other children, according to reports.

The victims — dual US-Mexico citizens with ties to Utah — were allegedly caught in crossfire of two cartels in the border state of Sonora, near where they worship with Church of Jesus Christ of Latter-day Saints.

The families lived in La Mora, a Mormon community about 70 miles south of Douglas, Arizona.

Kendra Lee Miller, a relative, identified the victims as Christina Marie Langford Johnson, 29, Dawna Langford, 43, Trevor Langford, 11, Rogan Langford, 2, Rhonita Maria Miller, 30, Howard Miller, 12, Krystal Miller, 10, and 8-month-old twins Titus and Tiana Miller.

Julian LeBaron said his cousin was traveling with her four kids to the airport when they came under fire.

“It was a massacre,” LeBaron, an activist who has denounced gangs in the area, told Formula Radio, according to AFP. His cousin’s van was set ablaze fire with the victims inside, he added.

Two other vehicles found several hours later contained the bodies of two women and a boy and a girl.

At least five other children — one of whom was wounded — managed to escape and walk home, and a girl was reported missing after having fled into the woods to hide, LeBaron said.

When asked who might be responsible for the bloodshed, LeBaron said the attack took place in a “war zone” — home to drug cartels and “thugs.”

The families “may have been caught in crossfire or targeted by mistake, we don’t know the cause,” he told AFP, adding that the Mormon community had recently been the target of threats.

The community are descendants of Mormons who fled the US in the 19th century after being persecuted for their traditions, including polygamy.

Many Mormons in Mexico enjoy dual Mexican and US citizenship and the LeBaron family asked for the help of the US Embassy.

As many as 13 other members of la Mora were initially reported as missing after the attack, a relative who asked not to be named told CBS News.

Another relative, Jhon LeBaron, posted on his Facebook page that his aunt and another woman were dead, and that said six of his aunt’s kids had been left abandoned but alive along a road.

He added that a total of 17 of his family members were in the three-vehicle convoy when it came under attack. He said six were wounded and two left unharmed.

Willie Jessop, a relative of one of the victims, told NBC News from Utah that the family has been trying to mobilize Mexican federal authorities and were in touch with the FBI.

“Everyone is in so much shock,” he told the network. “It’s just unbelievable and there’s just no way to comprehend it.”

El Universal, one of Mexico’s largest newspapers, quoted other family members as saying that their relatives were killed in what appeared to be an organized crime ambush.

The Mexican national civil defense agency confirmed that the National Guard, the army and the state police were conducting a search in the municipality of Bavispe on Monday night.

US Ambassador 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.”

Sonora Gov. Claudia Pavlovich Arellano tweeted 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 added.

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

It is not the first time the family has been targeted.

Julian Lebaron’s sibling, Benjamin, founder of crime-fighting group SOS Chihuahua was killed in 2009 in neighboring Chihuahua after he led protests over the kidnapping of their 16-year-old brother, according to AFP.

The Mormons refused to pay the ransom and the young Lebaron was ultimately released.