MEXICO CITY — Assailants killed at least nine members of a fundamentalist Mormon community in Northern Mexico, authorities said Tuesday, shooting and incinerating women and children in a brutal assault that highlighted the increasing grip of organized crime on parts of the country.

Mexican Public Security Secretary Alfonso Durazo said three women and six children of the extended LeBaron family were killed in attacks on three vehicles Monday in the northern state of Sonora. The victims were part of a community of U.S.-Mexican dual citizens who have lived in Mexico for decades.

Drug traffickers in the region came under immediate suspicion. But the massacre was barbarous, even for them.

“There have been conflicts between the cartels of Chihuahua and Sonora,” family member Julian LeBaron said on Mexico’s Radio Fórmula. “But to open fire in broad daylight on women and children? This crime has no name.”

The attacks on the women and children — some of whom were traveling to a wedding — stunned a nation reeling from a series of violent incidents in recent weeks. President Andrés Manuel López Obrador has been lambasted for the botched attempt last month to arrest a son of the former Sinaloa Cartel leader Joaquín “El Chapo” Guzmán. Soldiers briefly detained Ovidio Guzmán in Culiacán but let him go after cartel gunmen took control of the city.

President Trump offered to help Mexico strike back at the traffickers. “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,” he tweeted. He called López Obrador on Tuesday afternoon to offer support.

López Obrador thanked Trump but said pursuing the criminals was “a matter of sovereignty” for Mexico. The leftist leader has been a strong critic of U.S.-backed offensives against organized crime groups over the past 13 years, and has instead emphasized addressing the causes of violence with social programs.

“We don’t think that by opening fire, massacring, using force, blood and fire, we will resolve this problem,” the Mexican president told journalists.

But the shocking attack was likely to affect the country’s security policy, wrote Falko Ernst, the senior Mexico analyst for the International Crisis Group. “Over the next days, I’d expect pressure within the U.S. to build on [the] Trump [administration] — by media and evangelicals, e.g. — and for that pressure to be passed onto López Obrador,” he tweeted.

Mexican officials and some of the victims’ relatives said cartel gunmen might have mistaken the women’s SUVs for those of rival traffickers. But other family members said the attackers knew they were firing on civilians.

“There’s been a lot of rival cartels fighting up in this area,” said Lenzo Widmar, a cousin of several of the victims who helped search for their vehicles on Monday night. But he said a child who survived told family members that one of the women stepped out of her car and put her hands up.

“They shot her anyway,” Widmar told The Washington Post. “They knew it was women and children.”

More than 200 bullet casings were found near the vehicles, state authorities said.

Widmar said the community had not received any threats recently. He said all the victims lived in Mexico, though they have close ties to the United States, where they have family and many of the men do seasonal work. The LeBarons describe themselves as Mormons. They’re part of a polygamous offshoot of the Church of Jesus Christ of Latter-day Saints.

Widmar said the attacks began Monday morning after the families left the community of La Mora in Sonora state. One woman, Rhonita Maria Miller, was heading to Arizona to pick up her husband from the Phoenix airport, he said. The other two were going to accompany her as far as a main highway near the border and then head for Colonia LeBaron in nearby Chihuahua state, to attend a wedding. The community is named for the clan.

She was just outside the village of San Miguelito when her Chevrolet Tahoe came under attack on Monday morning, Sonora state security officials said. Gunmen shot her and her four children, including 8-month-old twins, relatives and officials said. The vehicle was then set on fire.

About 11 miles east, toward the Chihuahua state border, authorities found a second vehicle, a white Chevrolet Suburban, with the bodies of a woman and two children. Relatives identified them as Dawna Langford and her 11- and 3-year-old sons. They said several other children escaped from the vehicle.

The third vehicle, also a white Suburban, was found about a mile east of the Chihuahua border. The body of a woman was found nearby. She was identified as Christina Langford Johnson.

Family members said Dawna Langford’s 13-year-old son, Devin, watched his mother die and then hid his six surviving siblings in nearby bushes. Kendra Miller, the sister-in-law of Rhonita Miller, said the boy covered them with branches before he set out for La Mora for help. He reached the village at 5:30 p.m., she said, six hours after the attack, and relayed the news of the ambush of the women and children.

“For 11 hours, their families all over Sonora, Chihuahua and the midwestern US waited in fear and horror for any news of possible survivors,” Miller wrote on Facebook.

Relatives said they were stunned to learn that Johnson’s baby had survived the shooting.

“When they started shooting, she ducked down and put the baby behind the seat,” said Kenny LeBaron, a cousin who grew up in Mexico and now lives in North Dakota. “I think it was a miracle.”

One child is still missing, authorities said. Five of the surviving children were flown to Tucson for treatment.

In communities where so many were related by blood, marriage and friendship, residents were subsumed in grief. It rippled across the border to North Dakota, Arizona and beyond.

“I think we’re all still in shock, just trying to survive minute by minute,” said Leah Staddon, another cousin of Johnson who grew up in La Mora before moving to Queen Creek, Ariz.

About half of her siblings still live in Mexico. They stay in touch through WhatsApp. Staddon said the violence in the area where the LeBarons live seems to have grown worse in the past several months. Family members have been pulled over and threatened with guns.

“They said we would be okay as long as we didn’t travel at night. Everybody made sure they were traveling in the daytime,” she said. “But this happened yesterday in the middle of the day.”

A crescendo of violence in Mexico is posing one of the biggest tests of López Obrador’s year-old presidency. Last month’s failed anti-drug raid in Culiacán stirred so much concern that retired generals took the unusual step of publicly complaining to Defense Minister Luis Cresencio Sandoval about the president’s strategy.

López Obrador has created a National Guard with about 70,000 troops to improve security, and provided millions of young people with scholarships aimed at discouraging them from involvement in organized crime. But Mexico is on track for its deadliest year in recent history, with nearly 26,000 people slain in the first nine months of 2019.

The two U.S. senators from majority-Mormon Utah, Mitt Romney and Mike Lee, called on López Obrador to ensure justice after the massacre. Romney’s forebears lived in a different Mormon community in Mexico, one that retained closer ties with the main LDS Church.

“The U.S. must work with Mexican officials to hold accountable those responsible for this senseless violence,” Romney tweeted.

Durazo, the public security secretary, told reporters that “serious advances” had been made in the investigation into the killings, but did not provide details.

Mormons began to settle in Mexico and Canada in the 1870s and 1880s to avoid being prosecuted by the U.S. government over their practice of polygamy, according to Matthew Bowman, the chair of Mormon studies at Claremont Graduate University in California. The main LDS Church, headquartered in Utah, abandoned polygamy and began to crack down on its practice by excommunicating members. Offshoot groups such as the LeBaron family began to form in Mexico in the early 1900s.

For decades, the LeBaron clan lived quietly in farming communities, speaking both Spanish and English. But their relative wealth made them targets of extortion and kidnapping as organized-crime groups increasingly asserted themselves. In 2009, a prominent member of the clan, Benjamin LeBaron, 31, was shot dead near his community. He had publicly denounced the drug traffickers after they abducted his younger brother and demanded a $1 million ransom. The family refused to pay, and the killers said the murder was in retaliation.

The children who survived Monday’s attack and were flown to Tucson are “doing okay,” said Staddon, who drove to the hospital. A 10-month-old boy and an 8-year-old boy were recovering Tuesday after surgery. A bullet knocked away much of the 8-year-old’s jaw.

“They’re doing pretty well, considering,” she said. “I’m sure as things set in, it will get a lot worse for them.”

Kayla Epstein in Washington, Sarah Pulliam Bailey in New York and Gabriela Martínez in Mexico City contributed to this report.