US, Florida - The unmistakable sound of gunfire stirred the neighbors early Tuesday. Moments later, there was a knock at the front door.

They opened it to find three children from next door - at least one of them shot and wounded, authorities said. The neighbors called 911, grabbed a towel and tried to attend to the kids.

Then, they told deputies later, a calm voice in the darkness summoned the three children back to their home on Bright Avenue in Port St. John. It was their mother, 33-year-old Tonya Thomas.

They kids walked back obediently. And once inside, Thomas shot and killed them.

Brevard County Sheriff's deputies were just arriving at 4:54 a.m. Tuesday when they heard the shots that ended the lives of Joel, Jazlin and Jaxs Johnson. They were 12, 13 and 15.

Their oldest sibling, 17-year-old Pebbles Johnson, was stretched out on the front yard motionless when deputies arrived. They drove onto the lawn and loaded her into the back of a deputy's cruiser, but by the time they could get her to paramedics waiting nearby, she was dead.

When deputies approached the home again just after 5 a.m., a person with a gun they later realized was Johnson came out of the home, but quickly turned and went back inside.

At 5:26 a.m., one final shot rang out.

After killing all four of her children, Thomas shot herself to death, according to Sheriff's Lt. Tod Goodyear.

"It appears all these children were dead at or before the time deputies arrived," he said during a news conference. "Everything points to her."

It was the worst instance of multiple homicides to take place in Brevard County since the April 1987 shootings in Palm Bay involving William Cruse, according to the Florida Today newspaper. Cruse killed six people, including two Palm Bay police officers, during a shooting spree.

Detectives late Tuesday were still piecing together the scene but are no closer to understanding why Thomas opened fire.

"I'm a father. I've got kids. I cannot comprehend a parent doing this to their children...to call them back into the slaughter," Goodyear said. "It's beyond anything I've ever seen."

Goodyear said that around 3 a.m. Tuesday, Thomas sent a text message to a family friend saying she wanted to be cremated along with her children. But the friend didn't see the note until well after the shootings, detectives said.

Thomas and the children's father had recently separated and he was living elsewhere. He could scarcely take the news when deputies told him what happened, officials said.

Little is known about the family's history, although Goodyear said they had been called to the home previously for domestic disturbances.

The last time deputies responded was in April, when Jaxs was arrested on a domestic violence charge against his mother. But the specific details were not available Tuesday.

Thomas' mother Helen Wakefield declined to comment.