(CNN) Nine members of a family from a Mormon community were killed on the Mexican side of the border with the United States, and a relative of one of those massacred told CNN on Tuesday the family is no stranger to drug cartel violence.

This photo shows Howard Jackob Miller, Jr., Rhonita Maria Miller, infant twins Titus Alvin Miller and Tiana Griciel Miller, and Krystal Bellaine Miller, who all died in the attack. Howard Miller, the adult male pictured, is alive and was not present at the attack.

Kendra Lee Miller, whose sister-in-law Rhonita Maria Miller died in the attack, said, "Cartels have taken too many of our family members" and those killed Monday were "not the first."

Miller told CNN her family has recently been threatened by cartels over where it can travel.

Investigators believe three vehicles traveling between the states of Sonora and Chihuahua were ambushed by criminal groups Monday evening, Mexican authorities said.

The victims include three women, four small children and two infants, family member Alex LeBaron said from Mexico. All were dual US-Mexican citizens, he said.

The victims were "all shot while in vehicles while driving," LeBaron told CNN. Eight children survived.

"Women and children were massacred, burned alive," LeBaron said. "Mothers were screaming for the fire to stop."

'Long-standing tensions'

The LeBaron family's history of conflict with Mexican drug cartels may indicate the family was targeted, former Mexican Foreign Minister Jorge Castañeda told CNN on Tuesday evening.

"They had stood up to the drug cartels and they did have certain frictions either with the cartels or with neighboring communities over water rights," Castañeda said.

Christina Marie Langford, who died in the attack

Castañeda was Mexico's foreign minister for three years and left office in early 2003.

"Their long-standing tensions, and apparently the woman who was driving in the first car that was attacked was an activist. She was someone who was very active in her community, defending her family, her fellow members of the community against cartels, on the issue of water rights," Castañeda said, describing one of the three women killed, without naming her.

The former minister also said the larger LeBaron community had been receiving the protection of 90 federal police stationed around the community since 2011 because of tensions between the family and cartels.

Dawna Ray Langford with her son, Trevor, who both died in the attack

That protection was withdrawn to some extent by the current government earlier this year, according to Castañeda. It's unclear whether all 90 policemen were withdrawn or just some of them, he added.

In a news conference earlier Tuesday, Mexican Security Minister Alfonso Durazo said the attack may have been a case of mistaken identity of "conflicting groups in the area."

'They were driving together for safety reasons'

In each of the three cars attacked, a mother was driving children to see family.

"They were driving together for safety reasons," Kendra Lee Miller told CNN.

This photo of the Miller family shows Zack Miller, 2, who is alive; baby Titus Alvin Miller, 8 months, who is dead; mother Rhonita Maria Miller, 30, who is dead; father Howard Miller, who is alive; baby Tiana Gricel Miller, 8 months, who is dead; son Howard Jacob Miller Jr., 12, who is dead; Krystal Bellaine Miller, 10, who is dead; Tristan Miller, 8, who is alive; and daughter Amaryllis, 5, who is alive. The faces of the children who are alive have been blurred.

Rhonita Miller, 30, was driving with some of her seven children from La Mora, Sonora -- where her family's ranch is located -- to Arizona to pick up her husband, Howard, who was in the US for work, Kendra Lee Miller said.

The family was supposed to go to Kendra's wedding next week in La Mora, the bride-to-be said.

In a second car, 43-year-old Dawna Ray Langford was driving, Kendra Lee Miller said.

"Dawna was going to visit her family with her children in Chihuahua," Kendra Lee Miller said.

Another member of the La Mora community, 29-year-old Christina Marie Langford Johnson, was driving the third car and "was going to meet her husband and the rest of her children because they were all moving up to North Dakota."

Christina Marie Langford died in the attack.

At one point in their journey, Rhonita's vehicle had a flat tire, Kendra Lee Miller said. Rhonita and her children left her disabled car behind and then traveled back to La Mora with the other two women and their children to get another car.

Rhonita took another family car with her children and continued on the journey with the other two vehicles, Kendra said.

Kendra said her brother Andre was on the mountain road near La Mora when he saw a fire in the distance where the caravan of three cars was attacked.

"My brother was fixing Rhonita's broken-down vehicle, saw a fire, didn't think anything of it," Kendra Miller said.

"Then (he) saw the explosion, went to check it out, saw it was my mother's vehicle, full of bullet holes, completely ablaze," she said.

"My dad, Andre and couple of uncles and relatives went to check up on the vehicle. All they found was charred remains, ash and bones," Kendra Lee Miller said.

"There were remains inside and outside of the vehicle."

A family member said a 13-year-old boy who was unharmed in the attack walked about 14 miles for help , after hiding his bleeding siblings in the bushes and covering them with branches.

'We are a massive family'

The Mormons who were attacked appear to be members of a fundamentalist sect that is separate from the mainstream Church of Jesus Christ of Latter-day Saints, an LDS church spokesman said.

"We are heartbroken to hear of the tragedy that has touched these families in Mexico," spokesman Eric Hawkins told CNN. "From what I can tell, these were members of a polygamist sect, and not members of the Church of Jesus Christ of Latter-day Saints."

Kendra Lee Miller said the community's members descended from Mormon settlers, and that some members are still practicing members of the church.

Tiffany Langford, who also lost relatives in the attack, said she was not raised in the LDS church but is now part of it.

"I was born in Mexico, but mostly raised in the US," Langford said. "I was raised as a fundamentalist Mormon, not part of the church. I joined on my own along with a lot of my family about five years ago. The rest of my extended family was raised about the same as me, some fundamentalist, some are now LDS, depending on who you're talking with."

Family members describe themselves as part of a Mormon community of about 3,000 members, living in their own agricultural community in Mexico.

Generally, the community is fundamentalist. One of the victim's relatives said some members practice polygamy.

Despite some differences between individual beliefs, "we are a massive family," Langford said.

"We love and support each other no matter what our individual beliefs. I've never seen such a strength of unity and love in a family as large as ours. This is the tragedy of our lives."

The bodies of the nine victims killed have been returned to their family ranch, LeBaron said Tuesday.

Funerals are expected to take place within the next 48 hours.

7 injured children are now in the US

Seven children injured in the ambush were flown from Mexico to Douglas, Arizona, for transport to Tucson hospitals, LeBaron said.

Sonora state Gov. Claudia Pavlovich Arellano said she was outraged by the attack.

"As a mother I feel angry, scorn and a profound pain for the cowardly events in the mountains between Sonora and Chihuahua," she tweeted.

"I don't know what type of monsters would dare to hurt women and children. As governor, you have all my collaboration so that this doesn't remain unpunished and the responsible parties pay."

The US State Department is aware of the attack, an agency spokesperson said.

"When a US citizen is missing or passes away overseas, we engage with local officials at multiple levels and provide all appropriate consular assistance," the spokesperson said.

The FBI has offered to assist Mexican authorities in the investigation, an FBI official said.

A region marred by violence

In 2009, a member of the LeBaron family living in Chihuahua state was abducted and returned unharmed a week later. His brother, Benjamin LeBaron, became an anti-crime activist but he and his brother-in-law were killed two months later.

The number of killings in the country has soared recently.

Last year, Mexico witnessed its highest number of homicides -- 33,000. And 2019 is on course to break that record.

Just last month, 13 Mexican police officers were killed in an ambush in the western state of Michoacan.

Now, grief from the latest high-profile massacre has spread across two countries.