Osvaldo Rivera, 31, of Camden, was arrested Sunday afternoon and charged with Murder and Attempted Murder.
He is being held at the Camden County Jail and an arraignment is expected to be held Tuesday.
Detectives combed the house for clues Sunday evening, dusting for fingerprints and trying to build their case.
The gruesome killing happened around 2:00 a.m. Sunday at a home on the 900 block of Ware Street.
A nearby resident, Nakyta McCray, was able to call 911.
McCray says the 12-year-old victim rushed out of her home banging on neighbor's doors.
Her throat had been slashed.
"Dispatch asked me was the girl still alive. I went to the house and saw her sitting there with her throat cut open and barely breathing," McCray said.
Camden police arrived on scene.
Inside the home, they discovered the girl's younger brother was dead. The killer also slashed his throat.
Two other siblings including a teenager were also in the home; they, however, were uninjured.
"The older sister kept crying that the other two little kids were in the house. I went down to try to get the two little kids, but I seen a bunch of blood, called for officer and he walked in the house and said there was another victim, it was her little brother," McCray said.
The children's mother has been in the hospital recovering from surgery.
Investigators have not released a motive.
The family of the victims tells Action News the 12-year-old was also sexually assaulted.
The name of the suspect has not been released as he has not been formally charged.
Neighbors, meanwhile, are shocked by what happened.
"I can't sleep. It could have been anybody's child, not just her. My prayers go out to the family," said neighbor Karina Mejia.
The 12-year-old remains in critical condition in Cooper University Hospital
Sunday was her 12th birthday.
In earlier versions of this story, the names of the young victims were given, however, due to new details that were released Sunday night, 6abc.com is no longer identifying the children by name.