Her name was Serenity — something she barely knew in her short life.

Serenity Brown, just shy of her second birthday, was possibly beaten to death by her father, then sliced up and fed to a pit bull in July 2006, according to law enforcement sources and documents.

But authorities didn’t learn about the giggly toddler’s disappearance until five years later — and are still trying to close the ghastly cold case in which a body was never found and no suspect ever arrested.

City social workers discovered the child was missing during a 2011 visit, and cops opened an investigation into her mother’s claims about the horrors that went down in a Brooklyn apartment.

Little Serenity was listening to her mother, Paula Johnson, read a story inside their Canarsie apartment when, according to Johnson, her ex-con dad, Edward (Chuck) Brown, flew into a rage over the child’s laughter, law enforcement sources said.

A single punch to the chest left the helpless tot unresponsive, Johnson told investigators.

Johnson claimed Brown snatched Serenity’s limp body from the mom’s arms, carried his daughter into the bathroom, put her in the tub and filled it with water, sources said.

He allegedly fetched a knife from the kitchen, turned up the volume on the radio and told Johnson to stay in the bedroom before carving up the dead child, sources said.

Johnson told investigators Brown then fed the girl’s remains to a pit bull, a source said. An anonymous caller last year claimed the crime was covered up when Brown then executed the dog.

The start of the probe was delayed for five years, as investigators only learned about Serenity’s disappearance in October 2011.

Social workers visited the mom’s new address 5 miles away in Flatbush after a report that Johnson, 35, was keeping her 12-year-old son home from school.

While at the Coney Island Ave. home, the workers came to the horrible realization that Serenity — while listed as receiving public assistance — was nowhere to be found, records from the Administration for Children’s Services show.

Johnson initially claimed Serenity was with her paternal grandmother in Alabama before coming clean about her daughter’s gruesome demise.

The mother claimed Brown, 34, killed the child and “they had discarded the body,” according to the ACS documents.

Serenity’s paternal grandmother, Alfreddia Norris, told the Daily News that the NYPD came to her Alabama home looking for Serenity in 2011.

Norris, whose son lives nearby, said she hadn’t seen the baby since the girl was 3 months old.

“I’d like to know what happened to her,” said Norris. “Where is she?”

The grandmother defended her son against the homicide allegations, suggesting the mother was to blame.

“Why did she say that and then say that he had her down here?” Norris said. “She’s the one folks need to be talking to.”

Repeated attempts to reach Serenity’s parents were unsuccessful. Neither has been charged with a crime.

A source said Johnson was questioned in 2011 by police and she told them Brown killed Serenity because the girl was laughing too loudly.

Johnson told authorities Brown plucked Serenity from the bed and slammed her down on a couch in the apartment’s single bedroom, leaving the child stunned. He then allegedly punched the tiny tot in the chest. Her body went limp. Johnson said she put her ear to the baby’s chest but didn’t hear anything, so she tried CPR, the sources said.

ACS received an anonymous phone tip last March claiming that “the mother and father got rid of the child by feeding the child to a pit bull. The father then killed the dog,” agency documents show.

Johnson’s mother, now living in the Coney Island Ave. apartment visited by ACS in 2011, was reluctant to answer questions about her long-missing granddaughter.

“Yes, I feel sad about it,” said Cynthia Johnson, 53, at her apartment. “I don’t know nothing about this.”

She said she rarely saw her granddaughter because Brown, who went to prison in New York for attempted robbery in 2000 and was released a year later, wouldn’t let her daughter bring Serenity around.

By the time caseworkers visited the mom in 2011, the parents had split up and moved from the building where Serenity was allegedly killed.

But investigators have continued to search for evidence in the 70-unit apartment building with views of Jamaica Bay.

Heather Scipio, 58, who took over the lease in 2010, said detectives have been to her apartment five times.

“They would come in here, looking in this closet, in that closet, in the bathroom, on the floor searching for evidence,” she said. “There’d be two of them here each time, for about 30 minutes each time, hoping to find something. ... They told me what happened here, that a little girl died.”

She said cops paid extra attention to her bathroom, looking under the rug for evidence, and knocking on a living room wall to see if anything was inside.

“I said, ‘How are you going to find evidence after that long?’ ” Scipio recalled.

She said cops haven’t been back since last winter — around the same time ACS got the phone tip.

Some relatives said they have long wondered what became of Serenity.

A man who said Serenity was his cousin, but asked not to be identified, said he hadn’t seen her since she was an infant.

“She was a gorgeous baby,” he said.

Serenity’s great-grandmother, who asked not to be identified by name, wondered why the mom hadn’t told anyone what happened.

“Why didn’t she say anything?” asked the woman, who lives on Coney Island Ave.

She was reluctant to discuss the tragedy any further.

“That’s a dead issue,” she said. “It was years ago.”

With Chelsia Rose Marcius and Robin Rayne Nelson