Three years before 4-year-old Marchella Pierce was found starved, beaten and drugged, the city's own investigators said child-welfare workers had failed to protect the vulnerable.

New York City's child-welfare agency had overseen 11 cases in less than a year in which a child died after workers reported the child was living in a safe, clean home. In all but one, the 2007 investigation charged, the Administration for Children's Services did inadequate or incomplete work. The inquiry prompted major reforms, but no caseworkers were held criminally responsible.

New York prosecutors now say the reforms might not have worked. They took the rare step of charging two child welfare workers with homicide, saying their job failures _ and efforts to conceal them _ cost Marchella her life.

Experts say it will be difficult to convince a jury that the workers should be held responsible for the death of the child. They say the prosecution sends a strong message, but they also worry about what it will say to people who do a notoriously difficult job.

"The good is: Obviously, we are looking out for our children, and we want to hold people responsible when they fall down on the job," said Bruce Boyer, the Director of the Civitas ChildLaw Clinic at Loyola University Chicago. "But who is going to want to do this job if a co-worker ends up in jail?"

Child-welfare workers, like other civil servants, are immune from criminal responsibility for problems that arise within the normal course of their work.

But Brooklyn District Attorney Charles Hynes said Wednesday that the negligence of caseworker Damon Adams and supervisor Chereece Bell went beyond poor job performance: They are accused of doctoring reports to show more visits to Marchella's home than were made. They pleaded not guilty.

Marchella's mother was on ACS' radar after she gave birth to a baby boy who tested positive for drugs. The 4-year-old had been born premature with underdeveloped lungs. She had a breathing tube in her throat and spent much of her life hospitalized.

She was allowed home in February 2010 and was dead seven months later from battered child syndrome, Hynes said. Her mother has been charged with murder and grandmother with manslaughter; they have pleaded not guilty.

Adams was accused of adding at least five phony reports that said he had visited the home. Prosecutors say he couldn't have visited because if he had, he would have seen Marchella's very visible injuries.

"If it's true (the workers) falsified records, and as a result no one was supervising the home, that could show criminal negligence," said Columbia University Law School professor Jane Spinak.

In an eight-month stretch starting in October 2005, 11 children who had been under ACS' care died; another nearly drowned.

In the most notorious case, 7-year-old Nixzmary Brown was starved and beaten to death by her stepfather in 2006. Nixzmary's mother is serving a prison sentence of up to 43 years for failing to help her battered, malnourished child. Her stepfather is serving 29 years on a manslaughter conviction for delivering the fatal blow.

The investigation by the city Department of Investigation outlined a troubling pattern of lying, incompetence, carelessness and ill-trained caseworkers. It gave grim details about the children, whose deaths likely could have been prevented.

The department made a list of serious recommendations. Many were implemented, including hiring additional caseworkers, an increase in coordination with other city agencies and a new case-tracking system.

Since then, there have been more than 300 layoffs in other areas of the agency, and the city's contribution to its budget has faltered.

Hynes has ordered a special grand jury to consider evidence of systemic failure and determine whether the investigators' recommendations were followed.

Mayor Michael Bloomberg said Thursday that they generally were.

"Whether they are followed in this case ... it's an investigation, and I just can't answer," he said, expressing "100 percent confidence" in longtime ACS head John Mattingly.

The results of the charges in Marchella's case have been profound for ACS workers, said Faye Moore, the spokeswoman for their union. On a visit Thursday to the unit where the two worked, people were panicked and tearful, she said.

"This was a structural failure, a system failure, thanks to budget cuts, and other reductions in the agency," she said. "They are not responsible for that, and they are being housed with criminals. It isn't right."

The accused workers are being held on bail. They are in protective custody because, their lawyers argued, they are now being jailed with people they previously investigated.