Fernando Romero sat slumped in a lawn chair as he recalled the last conversation he had with his brother, who was stabbed to death — along with four of his children — early Thursday morning.

The previous afternoon, around 2 p.m., Romero’s brother Martin called to say he was back at the Loganville mobile home after a long weekend at the beach with the five children he was raising with their mother, Isabel Martinez.

“They (said) they are OK,” Romero said, quietly.

Some 12 hours after that call, police believe, Martinez, 33, embarked on an unspeakable rampage, killing four of her children and their father, one by one. She used a knife, according to the criminal arrest warrant. One of the couple's daughters survived, transported by helicopter to Children's Healthcare of Atlanta, where she was in serious but stable condition Thursday night, Gwinnett County Police Cpl. Michele Pihera said.

Nine-year-old Diana is expected to remain hospitalized for another two to three weeks, according to a GoFundMe page set up by a relative of Martin Romero.

A public GoFundMe site set up by family member identified the surviving child as Diana. The site says Diana remains in the hospital and is expected to stay there for another two to three weeks to recover from her injuries. The site is seeking donations to cover the family’s funeral and medical costs. Photo: GoFundMe Romero Family Funeral and Medical Fund.

Credit: HANDOUT

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Credit: HANDOUT

Her father, 33, and four siblings — 2-year-old Axel, 4-year-old Dillan, 7-year-old Dacota and 10-year-old Isabela Martinez — were already dead when police arrived at the family’s mobile home on Emory Lane around 5 a.m. Thursday. Martinez had called 911 at 4:47 a.m. requesting help.

Investigators have yet to say whether a confession followed, but Gwinnett Police Cpl. Deon Washington said Thursday that Martinez is the “primary suspect,” adding investigators are “not searching for anyone else.” Martinez has been cooperative, police said.

PHOTOS: Children, father stabbed to death in Loganville

MORE: Gwinnett slayings among deadliest involving kids

She is charged with five counts of malice murder, five counts of murder and six charges of aggravated assault. Also, an immigration hold was placed on Martinez, which means the government suspects she may be in the country without legal permission and could be eligible for deportation. Immigration officials declined to comment Thursday.

Martinez is scheduled to appear in court Friday morning.

“What prompts a person to take the life of such innocent children and her spouse is something we may never understand,” Gwinnett police said in a statement. “This is a horrendous crime not only for the victims but for the extended family, neighborhood and community.”

But filicide, the killing of one's own child, isn't that uncommon. According to a 2014 study by Brown University, it occurs about 500 times a year in the U.S. Data shows that 72 percent of the children killed were age 6 or younger, with mothers about as likely to commit such crimes as fathers.

Police weren’t saying on Thursday what motivated Martinez to allegedly commit such heinous acts. Andrea Yates, who confessed to drowning her five children in 2001, was later found to be suffering from severe postpartum depression and postpartum psychosis.

Susan Smith, the South Carolina mother who let her car roll into a lake with her two sons inside, craved a relationship with a former paramour who didn’t want to raise a family.

In Loganville, grief enveloped the otherwise quiet, close-knit cul-de-sac of mobile homes on Thursday as residents struggled to come to grips with what happened in their midst.

Gwinnett Police are charging Isabelle Martinez with murdering four of her five children and their father. The fifth child is hospitalized in serious condition. (Gwinnett Sheriff's Office)

Credit: Gwinnett Sheriff's Office

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Credit: Gwinnett Sheriff's Office

Fernando Romero had arrived to pick up his brother for work that morning only to find his small home surrounded by police cars. He was visibly stunned as he spoke to reporters.

Neighbor Silvestra Perez Rodriguez said Martinez was “a very good mom. My daughter is really sad because one of the daughters is a good friend.”

Others in this tight-knit, largely Latino neighborhood north of Atlanta spoke highly of Martinez. But they also said she had fallen into a deep depression after the recent death of her beloved father. Her sorrow was compounded when she was unable to attend his funeral in Michoacan, Mexico, they say.

“She used to be a calm, happy person,” said neighbor Pedro Ramirez, 15. “She invited us over to her house, had barbecues.”

“Now she’s yelling at people,” Ramirez said. “She’s just very upset.”

Another neighbor said he’d recently heard the couple inside the home arguing before Martinez emerged in tears, sitting forlornly on her front steps.

Rodriguez recalled seeing her at church two weeks ago. Martinez seemed “very sad,” she said.

Flowers and stuffed toys left on the doorstep of a home where four children and their father were stabbed to death are shown Friday, July 7, 2017, in Loganville , Ga. Isabel Martinez the children's mother and the man's wife has been charged with the crimes. (AP Photo/John Bazemore)

Credit: John Bazemore

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Credit: John Bazemore

The family had moved into the mobile home park, located just off Midway Road, earlier this year. They seemed to fit right in.

Fifteen-year-old Ivan Nava said he frequently saw Martinez’s children playing in the neighborhood.

They seemed happy, like normal kids, he said.

“They didn’t look like they were in any kind of danger,” Nava said.

Jim Hollandsworth said the children had recently enrolled in an after-school program he runs nearby. He didn’t know the parents but said their kids quickly adapted to their new surroundings.

“Everyone is in complete shock,” Hollandsworth said. “The last few weeks our staff has gotten to know these kids very well.”

The shock is unlikely to wear off anytime soon.

“When they told me about it I didn’t believe it,” Nava said. “This is a good community right here.”