Mexican President Andres Manuel Lopez Obrador met Monday with relatives of nine Mormon women and children who were massacred in northern Mexico last month to discuss progress in the investigation.
The November 4 killing of three women and six children from a breakaway Mormon community with dual US-Mexican nationality, caused shock on both sides of the border, and increased pressure on Lopez Obrador's government to show it is acting against brutal violence by drug cartels.
"They presented the progress they're making in the investigation and the cooperation between Mexico and the United States on the case," one relative who attended the closed-door meeting, Lenzo Widmar, told AFP.
"It made us feel reassured that an in-depth investigation is being carried out."
The victims were killed in a hail of bullets on a rural road between the states of Sonora and Chihuahua, a lawless region known for turf wars between drug cartels fighting over lucrative trafficking routes to the United States.
Eight children managed to escape, six of them wounded. One 13-year-old boy helped the younger ones hide, then walked 22 kilometers (14 miles) home to get help.
Mexican authorities, who are cooperating with the US Federal Bureau of Investigation (FBI) on the case, say the attack was carried out by the La Linea drug cartel.
Investigators say they believe the cartel mistook the families for members of a rival gang. But some relatives have rejected that theory, saying they believe they were deliberately targeted.
In the wake of the killings, President Donald Trump urged Mexico to wage "war" on its drug cartels with US help. He later vowed to add Mexican cartels to the US blacklist of terrorist organizations, a proposal Lopez Obrador rejected, saying "cooperation yes, interventionism no."
Lopez Obrador's meeting with victims' relatives came a day after prosecutors announced they had arrested "several individuals" over the case, without giving further details.
Widmar said the authorities had scheduled a follow-up meeting in one month's time.
"We want to know who did it -- the ones who did it and the ones who gave the order -- and why the order was given to commit such an atrocity," he said.