[Is Mexico Becoming More Violent? Our Journalists Answer Reader Questions After a Brutal Attack]

At a news conference on Wednesday, Mexican government officials offered few new details about the ambush and its motivations, saying that the investigation was continuing. But they determined that the ammunition used in the attack were .223 caliber cartridges manufactured in the United States by Remington and associated with AR-15 and M16 rifles.

Some 200,000 American guns cross the border into Mexico illegally each year, according to some estimates, landing in the hands of the criminal organizations that fight to control the multibillion drug trade to the United States and helping fuel much of the violence overwhelming Mexico. Mexico has been lobbying the United States for more than a decade to stop the flow.

Marcelo Ebrard, Mexico’s foreign minister, also said that because the victims were from families that have dual nationality and move frequently between the two countries, Mexican officials would provide their American counterparts with full access to their files as the investigation progressed and would seek the collaboration of United States law enforcement, including the F.B.I., if necessary.

Officials also provided some clarification about the timeline of events on Monday.

General Mendoza said the ambush began with an attack at about 9:40 a.m. on a Tahoe sport utility vehicle that was traveling well behind the other two vehicles and carrying one woman and four children. The Tahoe appeared to have caught fire from the impact of the bullets, he said, and all five passengers were killed.

The second attack, on the two Suburban S.U.V.s that were traveling together, occurred at about 11 a.m., some 11 miles farther along the road. One of the vehicles carried six children, two of whom were killed, and one woman, who was also killed. The other four children escaped.

The other Suburban carried a woman, who was killed, and four children, all of whom managed to escape.

On Tuesday, President Trump wrote in a tweet that this is “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.”