A Brooklyn man gunned down his toddler daughter in her bed Wednesday morning in a triple murder-suicide discovered by the baby’s grandmother, police sources said Wednesday.

Terrance Briggs, 27, who the sources said was a Bloods gang member, committed the unthinkable crime inside a fourth-floor apartment at Riverdale Osborne Towers in Brownsville.

The carnage was discovered by Patricia Green, 53 — the mom of Briggs and grandmother of his daughter, Laylay — when she returned home at 5 a.m.

Laylay’s mother, Olivia Callender, sobbed, “My baby is gone” at a candlelight vigil outside the Osborne Towers Wednesday evening. “She’s not coming back, my baby.”

Briggs shot his stepdad, Loyd Drain Jr., 57, and 16-year-old stepbrother, Loyd Drain III, before killing Laylay and himself, the sources said.

“He has a self-inflicted gunshot wound,” one source said.

All four were shot in the head and pronounced dead at the scene, police said.

Loyd Jr., who was married to Green, was shot to death on the toilet, while his son was discovered in bed with an Xbox nearby, according to sources.

Briggs and Laylay, who was born Christmas Day 2016, were in the same room. The little girl was wearing a diaper and found in the bed, while Briggs’ body was between the bed and a wall.

The apartment looked as if it had been ransacked, sources said.

“She was very upset and crying. It’s a horrible scene,” said one about Green, who lives in the apartment. “What would make anyone want to kill a 1-year-old?”

Shell casings were recovered at the scene and a .40-caliber gun was found in a bedroom where there were no bodies, police and sources said. Sources said Green, the grandmother, allegedly moved the gun to that room before cops arrived.

No suicide note was found and it’s unclear exactly when the shootings took place.

Police have been called to the home several times for domestic incidents — 12 of which involved Briggs. In August 2005, he was busted for threatening his stepfather with a loaded gun, sources said.

Briggs had 13 arrests for charges including petit larceny, assault and menacing related to the incident with his stepfather.

Callender, the baby’s mom, lives elsewhere, sources said.

A neighbor, who didn’t want to give his name, said Loyd Drain Jr. was known as Big Daddy because of his husky build.

“He was a retired transit worker. He was in the military, and he was one of the sweetest guys you can ever come into contact with,” the neighbor said through tears. “He would help anybody. He would give the shirt off his back, man.”

Another neighbor, José Valdez, said Briggs had a reputation for being violent. “Killing the whole family like this . . . there has to be something wrong,” he said.

Valdez said the younger Drain, who was friends with his sons, was a good student and played basketball at PS 41 across the street from the housing complex.

“He was a good person. He was humble, smart and he only focused on basketball and school and his family members,” said a pal, Juan Campos, 15. “He wanted to play basketball. He wanted to go to college [and] make his family proud.”

Green and the elder Drain had the “perfect marriage,” according to another neighbor, Vanessa Swain.

“I can tell you that Patricia loved her family.”

Additional reporting by Stephanie Pagones and Alex Taylor