The charge was dismissed in October 2011 after a witness recanted allegations that Thomas left Zahree alone in a car, Camden County Prosecutor Warren Faulk said.
"This was a one-witness case, and consequently we could not indict," Faulk said.
The Prosecutor's Office closely reviewed how the 2010 charge was handled after the murder-suicide, he said.
"If those same facts were presented to us again, we would have done the same exact thing again," Faulk said, noting that authorities properly notified the state Department of Children and Families of the arrest.
The agency took Zahree away from Thomas after the arrest and put him in the care of his maternal grandmother, Faulk said.
In a statement released Wednesday, the Department of Children and Families said Zahree "had previously been residing with relatives while his mother sought court-ordered treatment for substance abuse and mental health disorders."
Thomas regained custody in court in April.
"As with all child deaths due to suspected abuse or neglect, we will vigorously investigate the circumstances around the death of Zahree Thomas as well as his mother," the agency's statement said.
Officials confirmed that the agency had an open case and had been working with family members to support Thomas and her son.
The department has been under the supervision of a federal judge since 2003 as a result of high-profile lapses in case oversight.
On a rambling six-minute 911 recording obtained by several news media representatives, Thomas sounds incoherent and agitated. After telling a dispatcher that her boyfriend stabbed her son, she admits she stabbed him with a knife.
"You know what, I did it. I'm lying," she said, repeating the admission later in the tape.
When the dispatcher asks whether her son is bleeding, she answers: "He is, not much." She then says: "He's not bleeding anymore."
She tells another dispatcher that she didn't take her Prozac, "but I should have."
When police arrived at the Parkside rowhouse, in the 1400 block of Kaighns Avenue, they found the boy's torso in a first-floor room and Thomas barricaded in a second-floor bedroom. Authorities continued talking to Thomas on the phone but did not immediately force their way into the bedroom because they did not know whether she was armed.
After the brief phone negotiation, Thomas plunged the knife into her neck, causing an injury so severe that when police entered the room shortly afterward, she was dead. She had a stab wound to the chest as well, authorities said.
Evidence of drug use was found in the house, but it will be several weeks before blood test results are available, said Jason Laughlin, a spokesman for the Prosecutor's Office.
Faulk said that although Thomas admitted using drugs in 2010, there was not enough evidence to file drug charges then or substantiate child endangerment.
"The mother did not admit she abandoned the child," Faulk said. "If, in fact, she had left the child with a responsible adult, that does not constitute child endangerment."
On Nov. 28, 2010, Camden police found Thomas on the ground at 29th Street and Lincoln Avenue screaming incoherently, Laughlin said.
Thomas told authorities that she had been smoking marijuana and the hallucinogenic drug PCP in a nearby park when she "blacked out" and could not remember where she left her child, according to a complaint filed in Municipal Court.
As police were transporting Thomas to Cooper University Hospital, she yelled that she did not know where her baby was, Laughlin said. Police went to her address, then in the 1000 block of Lois Street. A neighbor was there with the boy.
Faulk said that in 2010, a neighbor told police she had found Zahree in a car but did not know where Thomas was. When investigators for the Prosecutor's Office tracked down the neighbor in October 2011 to present the case to a grand jury, the neighbor told investigators that police in 2010 got her story wrong. Thomas, she said then, asked her to care for the child while she ran an errand.
That's when prosecutors dismissed the charge for lack of evidence, Faulk said. His office has no record of contact with state child protection workers after Thomas lost custody of her son, he said.
Lynette Brown, who lives two houses away from the crime scene, said Chevonne Thomas moved in over the summer. "She would always sit on the steps and smoke," Brown said. "She would always have her baby on her lap."
Thelma Moore, who used to live in the neighborhood and was visiting Wednesday, said she had known Thomas for several years. She said the woman "just walked around and talked and cursed to herself." Thomas, Moore said, had been seeing a behavioral health therapist.
Another neighbor, Tayari Horcey, said Thomas appeared calm when she saw her sitting on her front steps by herself around 10:30 p.m.
"She just said good night," Horcey said.
Horcey added: "She could have cried out for help. I would have sure enough helped her."
Horcey said a woman, who appeared to be her late teens to early 20s and identified herself as Thomas' daughter, showed up at the house around noon.
Contact Barbara Boyer at 856-779-3838 or firstname.lastname@example.org.
This article contains information from the Associated Press.