MEXICO CITY, Nov. 5 (Xinhua) -- At least nine members of an American family, including three mothers and six children, were ambushed and killed by suspected members of a drug cartel in northern Mexico, officials said on Tuesday.

The victims -- all members of a Mormon family that live in northern Chihuahua state, but have U.S. nationality -- were travelling in at least two vehicles along a dirt road between Chihuahua and neighboring Sonora state when they came under attack Monday afternoon.

"The convoy ... could have been mistaken by criminal groups fighting for control of the region," Mexican President Andres Manuel Lopez Obrador said at his daily press conference.

The victims died in a hail of bullets and their vehicles were then set on fire. Another six children were wounded in the attack.

At the same press conference, Public Security Minister Alfonso Durazo said, "There has been serious progress in the investigation."

One group linked to the Pacific drug cartel is active in the area of the attack, according to the minister.

The adult victims have been identified as Rhonita LeBaron, and Dawna and Christina Langford.

U.S. President Donald Trump offered to help Mexico fight against drug-trafficking groups. He tweeted, "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!"

Lopez Obrador said he appreciated the offer but would not accept it. "This is a matter for us, the government of Mexico, to deal with independently."

In the wake of recent massacres, Lopez Obrador's long-term approach to combating violent crimes through education and poverty alleviation has come under fire, with some calling for tougher measures.