Drug cartel gunmen ambushed three SUVs along a dirt road, slaughtering six children and three women – all U.S. citizens living in northern Mexico – in a grisly attack that left one vehicle a burned-out, bullet-riddled hulk, authorities said Tuesday.

The dead included 8-month-old twins. Eight youngsters were found alive after escaping from the vehicles and hiding in the brush. But at least five had gunshot wounds or other injuries and were being treated in the U.S., where they were listed as stable, officials and relatives said.

One woman was killed after she apparently jumped out of her vehicle and waved her hands to show she wasn't a threat, according to family members and prosecutors.

Mexican Security Secretary Alfonso Durazo said the gunmen may have mistaken the group's large SUVs for those of rival gangs.

The bloodshed took place Monday in a remote, mountainous area in northern Mexico where the Sinaloa cartel has been engaged in a turf war. The victims had set out to visit relatives in Mexico; one woman was headed to the airport in Phoenix to meet her husband.

While a drug-related violence has been raging for years in Mexico, the attack underscored the way cartel gunmen have become increasingly unconcerned about killing children as collateral damage. Around the ambush scene, which stretched for miles, investigators found over 200 shell casings, mostly from assault rifles.

"Lately it's getting worse. This is a whole new level," said Taylor Langford, a relative of the dead who splits his time between the Mexican community and his home in the Salt Lake City suburb of Herriman, Utah.

In a tweet, President Donald Trump offered to help Mexico "wage WAR on the drug cartels and wipe them off the face of the earth." But Mexican President Andres Manuel Lopez Obrador rejected that approach, saying his predecessors waged war, "and it didn't work."