Many questions remain unanswered about the state's supervision of Thomas, who had a history of mental illness and drug abuse, in the last weeks of her life.
The department's commissioner, Allison Blake, did not return calls seeking comment.
On Friday, the department reported that Thomas tested clean for drugs June 29 and that a state worker last checked on her July 3.
"The case was reviewed again on July 23, 2012, and was being prepared for closure," according to the news release issued by Blake's director of communications, Kristine Brown.
"Our ongoing review does not reveal that any support systems involved with Ms. Thomas had any indications of relapse or noted any behaviors that could foreshadow harm to Zahree," the release said.
In November 2010, Thomas was charged with endangerment after she told police she had smoked marijuana laced with the hallucinogen PCP (phencyclidine), known to cause violence in some users, according to court records.
Police took Thomas, who could not recall where she left her child, to Cooper University Hospital for treatment, authorities said. A neighbor initially told police that she had found the boy alone in a car and that she did not know where Thomas was.
Nearly a year later, the neighbor recanted, saying Thomas had asked her to watch the boy while Thomas ran an errand, authorities said. Prosecutors then dismissed the endangerment charge.
Zahree was placed in his maternal grandmother's care, authorities said.
According to the news release issued Friday, state workers substantiated neglect, and Zahree was not returned to Thomas until July 11, 2011. Thomas had successfully completed counseling for substance abuse, mental health, and other issues.
On Aug. 2, 2011, however, Thomas tested positive for PCP in a random urine screening, and Zahree was removed again and placed with the same relative until April 3.
Nancy Parello, a spokeswoman for the nonprofit Advocates for Children of New Jersey, said that questions remained but that she believed there was a "good-faith" effort by the state to find answers.
"Obviously, this is a very complicated case," Parello said. "It is encouraging that they're releasing more information and there's a plan in place for a detailed review."
The state agency has been under the supervision of a federal judge since 2003 after high-profile lapses in oversight.
A recent report said that social-worker caseloads were rising and that only 55 percent of children placed by the state had two documented visits by caseworkers each month as mandated.
Thomas, a 1996 graduate of Northeast High School in Philadelphia, called 911 about 12:30 a.m. Wednesday and told a dispatcher that she stabbed her son. In a rambling six-minute conversation, she said she used the antidepressant Prozac but did not take it that day.
When police arrived at Thomas' Parkside apartment in the 1400 block of Kaighns Avenue, the boy's torso was found on the first floor and his head in the freezer. In a second-floor bedroom, Thomas had fatally stabbed herself in the neck, authorities said.
Although evidence of drugs was found at the scene, authorities said it would be several weeks before blood tests on Thomas were complete.
In Friday's news release, the Department of Children and Families said it would continue its review and release additional information.
"We owe Zahree and the Thomas family nothing less than a diligent, careful review of the facts, and we anticipate that process to take some time to ensure accuracy and thoroughness," the release said.
Contact Barbara Boyer at 856-779-3838 or firstname.lastname@example.org.