Three women and six children died in an attack on an extended Mormon family as they traveled in Mexico on Monday, Mexican officials say. All of those killed were reportedly U.S. citizens, and authorities say they might have been ambushed by gang members.

The victims are part of the LeBaron family, which has lived in Mexico for decades since splitting off from the Church of Jesus Christ of Latter-day Saints following its move away from polygamy.

"Local reports say the family may have been mistakenly attacked by rival cartels fighting for territory," NPR's Carrie Kahn reports from Mexico City.

Details have been slow to emerge about the attack on the group as it moved in a small convoy of vehicles between the states of Chihuahua and Sonora. Video from the scene shows that at least one vehicle was burned and had what appeared to be multiple bullet holes.

In addition to the nine dead, six children were injured and one minor was unharmed — and another minor is believed to be missing, President Andrés Manuel López Obrador and Security Minister Alfonso Durazo said at a news conference Tuesday morning.

In response to the attack, President Trump said, "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, including young children, and some missing."

Trump offered to aid Mexico if it needs help "cleaning out" the attackers, whom he dubbed "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. We merely await a call from your great new president!" Trump said via Twitter.

Mexico Foreign Relations Secretary Marcelo Ebrard says his agency has been in touch with U.S. Ambassador to Mexico Christopher Landau since Monday regarding the killings, which he called a tragedy.

"The security of our nationals is our top priority," Landau said via Twitter. He added, "I am following the situation in the mountains between Sonora and Chihuahua very closely."