President Andres Manuel Lopez Obrador says Mexico will cooperate with US officials in investigating the attack.

Mexican officials pledged on Wednesday to seek justice for the nine US citizens killed by gunmen in an attack that brought renewed attention to Mexico’s drug cartels, who are believed to be behind the ambush.

The three women and six children, members of a breakaway Mormon community who settled in northern Mexico decades ago, were killed on Monday after the three 4x4s they were travelling in was attacked.

Eight children were found alive hours later after escaping from the vehicles and hiding in the brush of a mountainous area of Sonora. Five children had bullet injuries and were transported to the US for treatment.

President Andres Manuel Lopez Obrador during his daily news conference on Wednesday said that he would cooperate with US officials in investigating the attack.

“We will ensure that there is an investigation and that justice served, and we have no limitation on informing you [the media] on how it is going. And if they (the US) want to participate, they can,” Lopez Obrador said.

Relatives of victims standing beside charred remains of a vehicle in Bavispe, Sonora state, Mexico [Jose Luis Gonzalez/Reuters]

Late on Tuesday, Foreign Minister Marcelo Ebrard visited the site of the attack where the Sinaloa cartel has been engaged in a turf war with rival gangs, and said that Mexico will take actions that are “decisive and of conviction” and equal to those expected from the US when it comes to Mexicans on their territory.

“That’s why we are here at this time, to be at the scene and to later have a meeting,” Ebrard said.

“The objective is for this indignation to be backed up by the law, for there to be justice,” he said.

In a statement posted on Facebook early on Wednesday, the Agency of Criminal Investigators in the state of Sonora said they had arrested a suspect in the town of Agua Prieta near the border with the US state of Arizona in connection with the attack. The suspect was found in possession of four assault rifles and ammunition.

Officials said the gunmen may have mistaken the family’s large 4x4s travelling together for those of a rival gang.

Austin Cloes, a relative of the LeBaron family victims who lives in Herriman, Utah, told local news outlet KGUN-TV said it is unclear whether the attack was deliberate.

“We don’t know if they were targeted or they’re just caught in the crossfire of kind of a war over territory between two cartels,” Cloes said.

“I mean, this sort of thing shouldn’t go unnoticed and these sort of people shouldn’t just be buried without their names being put out there. These are great people, these are US citizens,” he said.

Relatives of victims standing beside remains of vehicle in Bavispe, Sonora state, Mexico [Jose Luis Gonzalez/Reuters]

The LeBaron family is part of a Mormon community that broke away from the Church of Jesus Christ Latter-day Saints in Utah in the 1920s, after it outlawed polygamy. They later formed a sect in northern Mexico called the Church of the Firstborn.

A video shot by a relative after the attack showed the charred remains of the vehicle by the side of the road after the gas tank exploded. Investigators found more than 200 shell casings of assault weapons at the scene.

On Tuesday, US President Donald Trump offered to help Mexico wipe out drug cartels. He praised Lopez Obrador for combating cartel violence but said more needed to be done.

“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,” Trump said in a tweet.

Officials announced that Trump spoke with Lopez Obrador over the phone, and had offered to help ensure the perpetrators face justice.

Since 2006, Mexico has been waging a military crackdown on drug cartels that has led to the arrest and killing of dozens of prominent traffickers. But the campaign has not succeeded in reducing drug violence in the country and has led to more killings and to increased infighting among criminal groups.