Her father shot her in the head and murdered the rest of her family, but a brave 12-year-old Queens girl not only survived the bloody rampage — she managed to call 911 and open the door for police, too.

Christina Walker was in critical condition at Long Island Jewish Medical Center Saturday night after her dad fired at her head with a .45-caliber gun that resulted in the bullet exiting through her eye, sources said.

The girl was the only one alive after Jonathon Walker, 34, entered the Springfield Gardens home at 5:38 a.m. and marched from room to room, slaughtering his family in cold blood, police said.

Walker put a bullet in the head of his girlfriend of 10 years, Shantai Hale, 31; Hale’s mother, Viola Warren, 62; and little Kayla Walker, his 7-year-old daughter with Hale, ­authorities said.

The father fled the blood-soaked scene, calling a brother in Las Vegas about 20 minutes after his despicable deeds, police said.

“What I did, I cannot come back from,” the killer allegedly said.

Meanwhile, Christina called 911, waited for police and spoke to them for roughly 15 minutes before she was taken to the hospital, sources said.

It wasn’t long before the body of the ­6-foot-5, 260-pound nightclub bouncer and security guard was found in his car, with a self-inflicted gunshot wound to the head, a few miles away in a wooded area behind Kennedy Airport, police said.

It’s unclear what prompted the rampage.

“Nobody knows why,” said Wendell Warren, Viola’s brother. “We will be ­baffled for years.”

Records show that the family had two domestic incidents “of nominal issues” about 10 years ago, but nothing since then, said NYPD Chief of Detectives Robert Boyce.

Wendell Warren described Kayla and Christina as “the best kids you’ve ever seen in your life. I don’t know how he could take them away like that.”

Christina is a fighter. “She was always the toughest,” he added.

Another relative, Joseph Simmons, said doctors had to cut part of Chistina’s skull to ease the swelling in her brain. “We are hoping she comes out as Christina and not somebody else,” Simmons said.

Walker, a Buffalo native who played pro basketball in Portugal from 2008 to 2009, told a friend he was “going home to watch Netflix with the family,” said City Councilman Donovan Richards, who called the murder scene “truly horrific.”

“We’ve heard he was out with a friend. He told his friend he was going to get some beers and going home to watch Netflix with the family. We don’t know what made him snap when he came in,” ­Richards said.

Viola Warren worked at JFK, and Hale, whose brother is an NYPD ­officer, worked as a computer analyst, neighbors said.

Walker had just gotten a gun license ­“because he was going to go into private security,” said Wendell Warren.

“He drank but he had no drinking problem,” Wendell said of Walker.

“There was no indication of anything that could make him remotely do this, none whatsoever. It just came out of ­nowhere.”

The unspeakable violence left friends, family and neighbors on this dead-end block with the same question as voiced by Richards: “What could trigger such ­madness?”

Additional reporting by Jennifer Bain