The fatalities include six children after an ambush by gunmen. President Trump has urged his Mexican counterpart to accept help from the US "to wage WAR on the drug cartels and wipe them off the face of the earth."
Three women and six children were killed in drug cartel attack on a Morman family in northern Mexico, Mexico's security minister said Tuesday.
Alfonso Durazo said the gunmen may have mistakenly thought the group's large SUVs belonged to rival gangs. He also said six minors were injured in the ambush, five of whom were subsequently transferred to hospitals across the border in Phoenix, Arizona.
The victims were reported to have both Mexican and US citizenship.
Family members of the victims also suspected the incident may have been a case of mistaken identity.
According to Mexican media, the victims were members of the LeBaron family, from a breakaway Mormon community that settled in the country from the United States several decades ago.
More victims possible
The governments of Chihuahua and Sonora states, both of which lie on the United States border, issued a joint statement affirming that an investigation into the episode had been launched. According to the statement, there were still some people missing.
Local television showed a charred vehicle that may have belonged to the family. Local activist and relative Julian LeBaron was quoted as describing the incident as a "massacre," adding that some family members were burnt alive. He said on his Facebook page that the dead woman was named Rhonita Maria LeBaron.
Four of the children who perished included twin 6-month-old babies and two other children aged 8 and 10.
Trump offers support to rid Mexico of 'these monsters'
US President Donald Trump later tweeted: "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 then offered to lend Mexican President Andres Manuel Lopez Obrador his support. "If Mexico needs or requests help in cleaning out these monsters, the United States stands ready, willing & able to get involved and do the job quickly and effectively," he wrote, before adding: "The cartels have become so large and powerful that you sometimes need an army to defeat an army!"
The president concluded: "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!"
The offer of assistance was later rejected by Lopez Obrador, who said he did not welcome foreign intervention to deal with such cases.
The Mexican president has traditionally preferred a less aggressive approach as he believes confrontational policies by his predecessors only led to more violence.
jsi/rt (Reuters, AP, dpa)