At least nine U.S. citizens, including six children, were killed in a massacre in the Mexican border state of Sonora Monday, a relative to many of the victims told NBC News.

The dead included 8-month-old twins, said the family member, Kendra Lee Miller. Some of the eight survivors, all of whom are children, sustained serious injuries. Miller said a 9-month-old child was shot in the chest and a 4-year-old was shot in the back.

The attack was described by local media as a highway ambush. Willie Jessop, who is related to one victim, told NBC News by phone from Utah that the attack occurred on a motorcade consisting of several families, and that survivors at the scene told him that three cars were shot at and one was set on fire.

Download the NBC News app for breaking news

© Kendra Miller Image: Rhonita Miller and her twins Titus and Tiana

“Everyone is in so much shock," Jessop said, adding that he has been in contact with Mexican federal officials and with the FBI. "It's just unbelievable and there's just no way to comprehend it."

The victims were identified by Miller as Christina Marie Langford Johnson, 29, Dawna Langford, 43, and Trevor Langford, 11, and Rogan Langford, two-and-a-half.