As police were investigating the death of a 4-year-old Bronx boy yesterday, officials from the city’s child protective agency were testifying at a City Council hearing focused on systemic flaws exposed following the death of 7-year-old Nixzmary Brown last month.

As officials from the city’s Administration for Children’s Services testified about the agency’s shortcomings, the agency suffered another setback: the death of a child whose family, as with Nixzmary’s, had been the subject of a previous ACS investigation, officials said.

The child, Quachon Brown, was reported unconscious yesterday at about 3:30 a.m. in an apartment on Kossuth Avenue in Bedford Park, police said. He had marks on his body when police took him to North Central Bronx Hospital, where he was pronounced dead at 4:18 a.m., police said.

Police indicated that Quachon’s mother said a television had fallen on her son’s head Friday, but a law enforcement official said detectives believe the boy was assaulted. Detectives learned that the boy had suffered a fractured skill and had old and new bruises on his body, the official said. As of last night, no arrests had been made and police said they were questioning Quachon’s mother and her boyfriend.

Police Commissioner Raymond Kelly confirmed that ACS had investigated Quachon’s family in June 2005. At that time, Quachon’s mother allegedly left the boy and her five other children alone during a trip to Atlantic City. No charges were filed, the commissioner said.

At a hearing of the council’s Committee on General Welfare, the ACS commissioner, John Mattingly, said that in Quachon’s case, a previous ACS investigation had found the family’s home “to be in order,” and he promised a followup investigation.

ACS released its preliminary findings in the investigation of Nixzmary’s case. In a seven-page report, the agency listed numerous casework lapses. In one, caseworkers deemed unfounded reports of educational neglect on the part of Nixzmary’s parent despite her 46 absences from school. The report also details the repeated failure of caseworkers to gain entry into Nixzmary’s home despite repeated attempts to do so for several weeks.

“The fact of the matter is, we did not carry through these investigations the way they need to be in effective child welfare practices,” Mr. Mattingly said.

Lawmakers pressed ACS officials to explain the agency standards that allowed for Nixzmary’s death. As committee members called on Mr. Mattingly to answer their questions, they asked whether “automatic triggers” that flag high-priority cases, including caseworkers’ inability to gain access to the subject’s home, should be in effect.

“I don’t understand why if you are denied access there is any ambiguity at all,” Speaker Christine Quinn said. “Doesn’t an inability to gain access move the case to some red flag list?”

“What I hear in this case are roadblocks,” the chairman of the council’s Welfare Committee, Bill de Blasio, said. “Do we have a policy problem there?”

Mr. Mattingly laid out several planned reforms, including remedies for overburdened caseworkers and closer collaboration with police to obtain court orders to enter allegedly abusive homes. He also detailed a new Child Safety Task Force and the deployment of additional staff to an emergency service office.

Committee members yesterday said a bill to enhance accountability at ACS would be introduced at tomorrow’s scheduled city council meeting. A second bill to be introduced would cap caseworkers’ caseloads at 12.