MEXICO CITY — Nine U.S. citizens were killed Monday when their vehicles were ambushed by gunmen in northern Mexico, a brutal attack that prompted President Donald Trump to call for a “war” against Mexico’s increasingly powerful criminal groups.

The victims —three women and six children — were members of the LeBaron family, who are linked to a breakaway fundamentalist Mormon sect that has been based for many decades in a remote stretch of Mexico not far from the U.S. border.

Relatives of the dead posted a video showing the bullet-riddled and charred remains of an SUV in which one woman and her children were apparently burned alive.

The group was ambushed as they traveled in a caravan of three vehicles from the town of Galeana, Chihuahua, to Bavispe, Sonora, Mexican Security Secretary Alfonso Durazo said at a news conference Tuesday morning. Durazo said the assailants may have mistaken the victims for members of a rival cartel because they were traveling in large SUVs, which are favored by criminal groups. He said at least five children were taken to Phoenix for treatment.

Durazo said that Mexican security forces had located six other children and another woman who fled the attack, and that another minor was still missing. The victims who were found were all injured, he said, with one child having a gunshot wound to the back.

Durazo and Mexican President Andres Manuel Lopez Obrador said the motives of the attack were unclear. “It’s been a very violent area for a long time,” Lopez Obrador said. “We’re going to wait to see what the investigations say about what actually happened.”

Trump tweeted angrily about the ambush Tuesday morning. “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,” he wrote.

Trump said the U.S. was ready to assist “if Mexico needs or requests help in cleaning out these monsters.”

“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,” the president tweeted.

His response was a harsh rebuke of Mexico, where Lopez Obrador has repeatedly said he does not want war with criminal groups, saying the militarized strategy of his predecessors turned Mexico into a “graveyard.”

Mexico and the U.S. have long cooperated on security under the Merida Initiative, a multibillion-dollar partnership under which the U.S. has trained Mexican police and soldiers and pushed for other criminal justice reforms.

In recent weeks, amid criticism from other U.S. officials that Mexico does not have a cogent security strategy to fight rising violence, Lopez Obrador has said the U.S. must respect Mexico’s sovereignty and that “officials from other countries should not offer opinions about internal issues that only concern our government.”

He reiterated that point on Tuesday morning at the news conference, saying Mexico will act independently.

“We are very grateful to President Trump, or to any foreign government that wants to cooperate and help,” he said, “but in these cases we have to act independently, in accordance with our Constitution and our tradition of independence and sovereignty.”

Monday’s massacre is the latest in a string of extreme violent incidents in Mexico that have created the biggest crises yet for the young Lopez Obrador administration.

Last month, 13 people were killed in a botched federal operation to capture a leader of the Sinaloa drug cartel in the city of Culiacan. Mexican forces decided to release the leader, Ovidio Guzman, after the cartel laid siege to the city for several hours.

Earlier in the month, 14 state police forces were ambushed and killed in Michoacan state. In August, the bodies of 19 people were hung from a bridge and dumped nearby in another city in Michoacan. Also that month, 27 people burned to death when criminals firebombed a strip club in Veracruz, allegedly after its owners failed to produce extortion payments.

The region where Monday’s massacre took place has long been lawless.

It is a highly remote mountainous region that is mainly disputed by two criminal organizations: the Sinaloa cartel and La Linea, a criminal group linked to the Juarez cartel.

The Mormon presence in the region dates back to 1886, when the Church of Jesus Christ of Latter-day Saints began cracking down on polygamists and a sect that had embraced that tradition fled the U.S. and purchased 50,000 acres of land in the Mexican state of Chihuahua along the Piedras Verdes river.

In 1944, a member of that community, Dayer LeBaron, acting on what he said was a message from God, relocated his family about 35 miles south and established a breakaway Mormon community known as Colonia LeBaron.