A gunman stormed into a Las Vegas grocery store and killed the mother of his children and her new husband before being tackled by heroic bystanders, according to police.

Alejandro Estrada, 43, had multiple weapons on him as he entered a Smith’s grocery store Tuesday morning and shot ex Amanda Rosas and her new husband Victor Frias Rosas, who both worked at the store, KLAS reported.

Estrada owed more than $2,800 in child support payments for their two kids — including a son who turned 12 on the day his mom was murdered.

“He was completely calm. No expression. No nothing,” shopper Paula Milton told the Las Vegas Review-Journal of the double murderer. “You could tell it was him because everyone else was running and screaming, and he was like no big deal … “

The accused killer was divorced from Amanda Rosas but still in the middle of a bitter child custody dispute over their two kids, the 12-year-old birthday boy and a 9-year-old girl, according to police and court records. Neither child was there at the time.

The shooting only ended when multiple bystanders jumped on top of the suspected shooter and held him down until police arrived and hauled him away in handcuffs, cops and witnesses said.

“Those individuals showed tremendous courage and did an outstanding job,” Metropolitan Police Department Lt. Robert Price told reporters at a press conference, confirming that the suspect was having “some custody issues” with one of the murdered workers.

Police said the gunman had multiple weapons on him. He was taken to a local hospital and treated for minor injuries.

Amanda and Victor Frias Rosas, who were married last year, were declared dead at the scene.

Estrada is facing 12 charges, including two counts of open murder. He is scheduled to appear in court Wednesday afternoon.