Mexican police have arrested a suspect in the massacre of nine US Mormons, as it emerged that a 13-year-old boy travelling in the ambushed convoy hid his siblings and walked for six hours across the desert to raise the alarm.

The unnamed suspect was holding two bound-and-gagged hostages in the northern Mexican hills of Agua Prieta in Sonora state, the ministerial Agency for Criminal Investigations (AMIC) said.

The suspect had several rifles and a large amount of ammunition, including a number of large-calibre weapons.

Cesar Peniche Espejel, attorney general for the neighbouring state of Chihuahua, confirmed the arrest in an interview with Mexico’s Imagen Radio but could not confirm details of the suspect’s involvement.

"These very cartels of Sinaloa, after the arrest of Guzman ‘El Chapo’ have suffered fragmentations," he said.

Earlier on Tuesday a US official said a rival cartel called La Línea is under focus.

Mexico's president, Andres Manuel Lopez Obrador, was elected on a pledge of "Hugs, not Bullets". That is proving a tough promise to keep

News of the arrest came as shocking detail of Monday’s events was brought to light.

Rhonita LeBaron, 30, was driving one car and was killed alongside her children Howard, 12; Krystal, 10; and eight-month-old twins Titus and Tiana.

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Dawna Ray Langford, 43, was driving a second car. Her sons Trevor, 11, and Rogan, aged two, were killed, as was passenger Christina Marie Langford, 29.

A six-month-old baby, Fe Maria, the daughter of Christina Langford, was found unharmed, hidden on the floor of the car.

It also emerged that one of the children, 13-year-old Devin Langford, witnessed his mother being murdered and escaped to raise the alarm – walking 13 miles, for six hours through the desert. He hid his six siblings before setting off to seek help.

Andres Manuel Lopez Obrador, the president of Mexico, has vowed to bring those responsible to justice.