MEXICO CITY (Reuters) – Mexico’s president said he would discuss security on Tuesday with the United States after President Donald Trump urged Mexico to “wage war” on drug cartels following the killing of nine members of a U.S.-Mexican Mormon family in the north of the country. The victims were three women and six minors, and President Andres Manuel Lopez Obrador said he would speak with Trump about possible cooperation on security, provided Mexico’s sovereignty were upheld. But he suggested he did not believe that foreign intervention was necessary. The dead belonged to the LeBaron family from a breakaway Mormon community that settled in the hills and plains of northern Mexico decades ago. Trump tweeted early on Tuesday that he would await a call from Lopez Obrador, urging him to accept U.S. assistance. “The great new President of Mexico has made this a big issue, but the cartels have become so large and powerful that you sometimes need an army to defeat an army!” he wrote. “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. Mexico has unleashed its military in a war on drug cartels since 2006 but despite the arrests or killings of leading traffickers the campaign has not succeeded in reducing drug violence. In fact, it has led to more killings as criminal groups fight among themselves. The government has registered more than 250,000 homicides in the last dozen years, most of them related to the drug war. When asked about Trump’s offer of help, Lopez Obrador said he would welcome help but did not want Mexico’s independence compromised. “I’ll speak with President Trump to thank him for his support, and to see if in cooperation agreements there’s the possibility of getting help,” he told a regular news conference. “I don’t think we need the intervention of a foreign government to deal with these cases,” he added. The government is investigating the motive for the killings, which took place on Monday on a dirt road between Chihuahua and Sonora states, both bordering the United States. Mexico Security Minister Alfonso Durazo said there was a possibility of mistaken identity, given the high number of violent confrontations among warring drug gangs in the area. “The convoy made up of suburban vans could have been confused with criminal groups that fight for control in the region,” Durazo said alongside Lopez Obrador. A video posted on social media showed the charred and smoking remains of a vehicle riddled with bullet holes that was apparently carrying the victims when the attack happened. “This is for the record,” says a male voice in an American accent, off camera, choking up with emotion. “Nita and four of my grandchildren are burnt and shot up.” Reuters could not independently verify the video. Five children were also injured and transferred to hospitals in the United States, U.S. and Mexican authorities said. Mexico has suffered a series of attacks in recent weeks, shocking even for a country inured to years of drug war violence. The most notable incident was a military-style cartel assault that forced the government to release a leader of the Sinaloa Cartel in October. “WE DON’T KNOW WHO DID IT” Chihuahua and Sonora state governments issued a joint statement saying an investigation had been launched and additional federal and local security forces were being sent into the area near the border between the two Mexican states. A relative of the family, Julian LeBaron, described the incident as a “massacre,” saying some family members were burned alive. In a text message, he said other injured members of the family were being transported to Phoenix, Arizona for treatment. He said four boys, two girls and three women were killed. “We don’t know why, though they had received indirect threats. We don’t know who did it,” he said. Several children who fled the attack were lost for hours in the countryside before being found, LeBaron added. “My cousin was murdered with her children in the truck,” said Alex LeBaron, another relative. In 2010, two members of the Chihuahua Mormon community, including one from the LeBaron family, were killed in apparent revenge after security forces tracked drug gang members. The Mormons had suffered widespread kidnappings before that. U.S. Ambassador to Mexico Christopher Landau, who travelled to Sonora earlier on Monday for unrelated work meetings, said he was following the incident closely.